Sanders is close to defeat. Here’s the math.

In the last week, Joe Biden has experienced one of the most extraordinary comebacks in the history of American presidential politics, and the nomination he has pursued for more than 30 years now lies within his grasp.

His remaining rival, Sen. Bernie Sanders, now confronts an achingly familiar situation: Just like four years ago, when he ran against Hillary Clinton, he’s behind in the delegate count, needing big wins to even the score, but with few obvious places to find them.

Unless Sanders can quickly pull off a comeback of his own in what has become a two-man race, the Democratic presidential contest could effectively be over by a week from Tuesday, when Arizona, Florida, Illinois and Ohio hold primaries.

As Janet Hook wrote in her analysis of Tuesday’s results, the issue for the next couple of weeks is whether either candidate can expand his existing coalition.

Let’s take a look, first, at the key moments of Biden’s recovery, then at the delegate math that makes Sanders’ position so challenging.

A decisive debate, a key endorsement and a wave

When the Democratic National Committee announced, on Jan. 31, that it was changing the rules on qualifying for its mid-February presidential debate, a lot of people cried foul.

“That’s the definition of a rigged system,” said Jeff Weaver, Sanders’ senior advisor.

“They shouldn’t change the rules to let a billionaire on,” said Sen. Elizabeth Warren.

The object of their scorn was Michael R. Bloomberg. The DNC had dropped the requirement that debate participants raise money from donors, allowing the billionaire former New York mayor, who was funding his own campaign, onto the stage.

To this day, some people believe the party bent its rules that day to favor Bloomberg. Nothing could be further from the truth. Allowing Bloomberg onto the debate stage marked the beginning of the end for him and the start of Biden’s revival.

Bloomberg by that point already had spent more than $400 million in pursuit of the nomination, creating a public image through massive television advertising. But as reporters who had covered him as mayor frequently said, Bloomberg live was nothing like the charismatic character created by the ads. The contrast made exposure on the debate stage perilous — the equivalent of the famous curtain scene in “The Wizard of Oz.”

When the event took place in Las Vegas, watched by a record television audience, Warren quickly landed rhetorical punches that crumpled Bloomberg’s candidacy. He did somewhat better in a second debate one week later, but not nearly enough to recover.

Bloomberg’s downfall coincided with Sanders’ rise. The Vermont senator triumphed in Nevada’s Feb. 22 caucuses, vaulting to the lead in the Democratic race.

Those two developments caused consternation among Democratic elected officials and many rank-and-file voters who saw Sanders’ victory as a fearful problem that would hand President Trump four more years in the White House. With the prospect of Bloomberg as a well-heeled savior evaporating, they were desperate to get behind an alternative.

To take advantage of that desperation, Biden needed to show he could win somewhere. South Carolina would be the place.

For weeks, Biden and his aides said black voters, the most loyal constituency the Democrats have — and the ones who most urgently feel that defeating Trump is a matter of survival — would stand with him. Heading into the week of the South Carolina primary, that confidence seemed questionable. Polls suggested Biden’s South Carolina wall might crumble.

Tom Steyer, the California philanthropist and political activist, had spent huge sums of his own fortune to advertise in the state with messages aimed directly at black voters. Maybe he would draw away support. Perhaps Pete Buttigieg or Sen. Amy Klobuchar or Warren could break through to a black audience.

Whatever prospects any of them had, however, ended abruptly on the morning of Feb. 26 when Rep. James E. Clyburn, the leading African American political figure in South Carolina, endorsed Biden. Clyburn’s words reassured wavering voters, and three days later, Biden triumphed, pulling in 49% of the vote in a multi-candidate field.

Of equal importance, the results confronted Buttigieg and Klobuchar with a stark reality: Between the two of them, they had received only 4% of the black vote in the state, according to the exit poll. For a Democratic presidential candidate, that’s not tenable.

The next day, all the major candidates except Sanders gathered in Selma, Ala., for the annual commemoration of one of the signal battles of the civil rights movement of the 1960s, the Bloody Sunday attack by state troopers and sheriff’s deputies against peaceful marchers at the city’s Edmund Pettus Bridge.

As they watched Biden receive cheers, Buttigieg and Klobuchar were already thinking through their withdrawal. Buttigieg would drop out of the race that night, Klobuchar the next morning. Both endorsed Biden on Monday in hastily choreographed events in Texas, where former Rep. Beto O’Rourke also came on board.

Individual political endorsements don’t often carry a lot of weight, but the wave of them sent a clear signal to voters that they now had a simple choice, Biden or Sanders. From Maine to Texas, the larger share chose Biden, even in states in which he hadn’t aired a single ad or opened an office.

Sanders won his home state, and Colorado, Utah and California. Even in California, however, his success illustrated his limits. In our final Berkeley Institute of Governmental Studies poll, which was conducted before the South Carolina vote, Sanders got 34% of California voters. That gave him a 2-1 lead in a crowded field in which Biden was in fifth place.

A week later, Biden sits in second place in California, with 25%. Sanders stands at 34%, exactly as the poll forecast. In all the turmoil, he appears not to have gained an inch.

Sanders’ steep road forward

Biden currently leads Sanders in the delegate count, 627-551. The now-suspended candidacies of Warren, Buttigieg, Klobuchar and Bloomberg have 157 delegates who are now, effectively, free agents.

There are 2,642 pledged delegates left to allocate, including several hundred from states that voted Tuesday where officials are still tallying final votes. California, for example, is only about halfway through the vote count, with more than 3 million ballots left to process, so only some of the state’s 415 delegates have been allocated.

To get to the 1,991 delegates needed for the nomination, Biden must win just under 52% of the remaining delegates. Sanders would need just short of 55%.

That doesn’t sound like a big gap. In reality, it’s a chasm.

The problem for Sanders is that each primary he loses will put that 55% goal further out of reach, and barring a sudden change in fortune, he’s on track to lose a bunch. His best prospects — states with large numbers of white liberals and Latinos — have mostly voted already, and many of the remaining states are not promising for him.

On Tuesday, for example, Mississippi joins Missouri, Michigan and Washington state in holding primaries. Mississippi’s Democratic electorate looks a lot like South Carolina’s and Alabama’s, which voted this week — heavily African American and not particularly sympathetic to Sanders’ brand of Democratic socialism.

Biden took 63% of Alabama’s vote and won 84% of its delegates since several candidates received too few votes to win any. In South Carolina, he took 72% of the delegates. Mississippi’s results almost surely will fall in that range.

In Missouri, Sanders almost caught Clinton four years ago. But that’s largely because he won nearly half the voters who called themselves moderate or conservative, according to the exit poll. Against Clinton, Sanders did well with conservative voters in a number of states, and in Missouri, they made up for black voters in St. Louis and Kansas City who sided with Clinton.

But those weren’t pro-Sanders voters, they were anti-Clinton, and in primaries so far this year, Sanders has done poorly with moderate and conservative voters.

Figure Missouri for another Biden win.

Washington could be a good state for Sanders, but it’s hard to know. Four years ago, the state held a caucus; this year, it’s a primary, which will probably have much higher turnout.

Sanders did very well in 2016 in states that held caucuses, which tend to reward candidates who have support from highly motivated activists, but he pressed the DNC to eliminate caucuses this year in favor of primaries. That was a praiseworthy step toward greater democratic participation, but it may hurt his chances further.

That leaves Michigan as the big test for Tuesday. Sanders won the state in an upset in 2016. He’s changed his schedule to add additional time there this weekend. If he can win it again and get a big majority of its 125 delegates, he can keep his hopes alive.

But Biden has support of the state’s popular Democratic Gov. Gretchen Whitmer — Sanders campaigned against her in the Democratic primary in 2018 — and several of the state’s members of Congress. If Biden can win the state, he could put the nomination almost out of Sanders’ reach.

Who’s stronger against Trump?

The question of electability has dominated the Democratic primaries like no other issue this year. Supporters of both Biden and Sanders argue their man is more likely to beat Trump.

Hook and Evan Halper examined the evidence and note that both would face stark challenges. Sanders has failed so far to generate the big turnout upsurge he has said his political revolution would need. As he admitted this week, turnout by young voters has lagged. And he’s shown weakness among the suburban voters who helped power the Democratic victory in the 2018 midterm elections.

Biden has shown considerable strength in the suburbs. But he’s weak among young voters and Latinos, both important constituencies for Democrats.

Meantime, as Jenny Jarvie and Mark Barabak wrote, another key constituency, politically engaged women, suffered another disappointment as Warren bows out.

Republicans hope to battle back in Congress

Republicans lost seven congressional seats in California in 2018, but they hope to regain at least a few this time around. So far, Sarah Wire wrote, their efforts are off to a decent start in early returns from two districts — the 21st in the Central Valley, where former GOP Rep. David Valadao holds a strong lead over Democratic Rep. T.J. Cox in the top-two primary, and the 39th, where Young Kim holds a slight lead over Rep. Gil Cisneros in a district that straddles the L.A.-Orange County line.

As Republicans know from bitter experience, Democrats tend to do better in election day and late-arriving mail ballots than in early voting. So we’ll have to wait to see if those early leads hold up.

Similarly, in early returns from the race to replace former Rep. Katie Hill in the 25th District, in northern L.A. County, the Republican Mike Garcia is in decent position heading into a runoff against the Democrat, Christy Smith.

Supreme Court generates election issues

For the third time, the Supreme Court will hear a challenge to Obamacare, with a hearing that could raise the profile of the healthcare law just before the election. As David Savage wrote, the justices agreed to hear an appeal from California Democratic officials seeking to rescue the law from a lower court decision that challenged its constitutionality.

The high court has twice before upheld the law, and the betting is it likely will do so again. But a hearing in the case this fall could focus attention on the Trump administration’s efforts to invalidate the law.

Before that happens, the justices are likely to issue a ruling on Louisiana’s abortion law. At a hearing this week, the court’s five conservative justices sounded likely to uphold the law, but along narrow lines that might not attract much political attention in an election year, Savage wrote.

Chief Justice John G. Roberts Jr. issued a statement that criticized Sen. Charles E. Schumer for attacks by name on two justices, Neil M. Gorsuch and Brett M. Kavanaugh. Schumer apologized for his comment in which he said the two justices had “released the whirlwind, and you will pay the price” because of their stands opposing abortion rights.

The other election wild card: The virus

The coronavirus has caused widespread anxiety, bordering on panic in some places. And it’s hurting the economy. That has agitated Trump, who fears the virus could hurt him politically. The real problem for Trump, Noah Bierman wrote: He can’t make a deal with a virus.

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