Though voters in dozens of states have yet to cast ballots and Sanders has amassed a significant campaign war chest on the strength of his grass-roots appeal,
Even as Clinton was sweeping to victory in delegate-rich states Tuesday, building an advantage Sanders is increasingly unlikely to reverse, some progressive groups began to realign their messages.
MoveOn.org, which has formally endorsed Sanders, spent as much of its statement on Tuesday's primaries warning about the threat posed by Trump as it did praising the potency of Sanders' message.
"If Trump is the Republican standard-bearer, it will be crucial for progressives, and all Americans, to unite to defeat a man who represents the antithesis of everything our nation stands for," said MoveOn's executive director, Ilya Sheyman.
Another group that styles itself as representative of liberal Democrats, the Progressive Change Campaign Committee, said Sanders had made Clinton a stronger candidate. The characterization seemed to imply that Sanders' challenge had served its purpose by putting Clinton clearly on record in support of the issues that motivate their activist base.
Adam Green, co-founder of the committee, said Sanders had helped ensure that "the center for gravity in the Democratic Party has shifted" to the left -- to what he called its Elizabeth Warren wing, after the Massachusetts senator. He said Sanders' challenge pushed Clinton to take more definitive positions on issues like Wall Street reform than she might have otherwise.
"On the one hand, it's a blessing to the overall Warren wing of American politics. But it also has blunted the distinction that Sanders might have had," Green said.
Clinton returned to New York on Wednesday to celebrate what she called a Super Tuesday "for the history books." But she stopped short of claiming she had the nomination in hand even as she looked ahead to a general election battle – promising she would wage a campaign "about the future."
She left it to surrogates to target Trump directly, but reprised her call for "more love and kindness" in the country.
"If we do what we must in this election to bring out a positive message of what we can do together, who we stand for and what we stand for, we will go into the November election with the wind at our back," she said. "And if I'm so fortunate to be your president, we will work together to make it true."
Sanders' campaign also brought up Trump on Wednesday as it insisted the senator from Vermont still had a path to the nomination.
Clinton would be vulnerable in a campaign against Trump, campaign manager Jeff Weaver warned. He referenced polls that show voters have doubts about Clinton's honesty.
Integrity will be a key issue in the campaign, Weaver said, adding, "You never get to your message if you're always trying to convince people you're honest."
"Our plan is to win, and win consistently, between now and June," Sanders advisor Tad Devine added.
But Sanders’ hope of fighting through to the end of primary season, including California’s June 7 primary and its massive haul of delegates, sounds like “more of a fantasy,” said Douglas Herman, a Los Angeles-based Democratic political consultant who worked on
"This guy is in the final stages of the 'Sixth Sense' movie," Herman said. "He doesn't know his campaign is dead."
"The call for unity against Trump is going to be so important, I don't see how Bernie can keep staying out there and saying he's a viable candidate," he added.
Democrats' sense that they must unite was evident beyond Tuesday's presidential primaries.
Obama made unusual endorsements in two key contested Senate primaries Wednesday, Florida and Ohio, states where party unity in the fall will be essential across the Democratic ticket.
It's a particular concern as turnout in the Democratic primaries has been dwarfed by that on the Republican side and has lagged well behind record-setting numbers in the 2008 nomination race.
White House Press Secretary Josh Earnest said this week there were no plans for Obama to endorse in the nomination fight, though he will cast an absentee vote in the March 15 Illinois primary.
"Both Secretary Clinton and Sen. Sanders – regardless of the final outcome here – have succeeded in motivating a significant portion of the Democratic Party to support their campaign, and the unity of the Democratic Party will be critical to our success in the general election," Earnest said.
Green said that even as progressives begin to consider the prospect of a general election campaign, what has been good politics for the nominee in the primary will continue to be good politics in the fall.
"If the net effect of Bernie Sanders staying in is that Hillary Clinton is speaking even more convincingly on the need to hold Wall Street accountable, that only helps her in the general election," he said.
Green suggested that Warren herself might soon be ready to make an endorsement – one Clinton would very much like to have. Groups on the left have joined Warren in calling on the eventual nominee to commit to naming strong progressive figures to key posts on the
"Her eventual support will be so important that she has the ability to make concessions particularly on Wall Street issues," Green said. "Elizabeth Warren is very good at picking her battles and picking her timing."
Staff writer Chris Megerian in Miami contributed to this report.
For more campaign coverage, follow @mikememoli
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