The vice presidential punch line?

It was a largely somber prime-time news conference, focused on recession and war -- except for a moment when President Obama slipped off message and turned his No. 2, Joe Biden, into a punch line.

A reporter asked the president to explain something Biden had said in a recent speech: Even if the White House did everything right in combating the economic downturn, the vice president said he had told Obama, there was still a “30% chance we’re going to get it wrong.”

What exactly did Biden mean by that?

“You know,” Obama replied Monday night in a light-hearted tone, “I don’t remember exactly what Joe was referring to.”

Then, the president added, “Not surprisingly” -- a seeming reference to Biden’s well-known gaffes.

Aides say Obama and the vice president eat lunch together once a week.

The two men are quick to praise one another in public. But a few episodes suggest the mutual admiration may have its limits.

Speaking at a security conference in Munich last week, Biden made a curious comment about the Obama administration’s $800-billion-plus stimulus package.

At a time when Obama has made the stimulus his central focus, when he is racing around the country, staging campaign-style events to build support for the plan, Biden referred to it as “our so-called stimulus package.”

That is not the sort of talking point the White House is putting out these days.

Obama and Biden’s rapport seems to be a work in progress. The president could barely conceal his annoyance last month when Biden attempted a joke at the expense of Chief Justice John G. Roberts Jr.

In giving Obama the presidential oath of office, Roberts had stumbled over the words.

The following day, Biden had a forgetful moment while swearing in some Obama administration officials.

“My memory is not as good as Justice Roberts’,” Biden said.

At that, Obama reached out his arm, as if to grab Biden and choke off any more quips.

A foreshadow of presidential impatience came during the campaign.

In October, Biden warned that if Obama went on to win the election, he would face a severe test from hostile interests within six months.

Obama tried to defuse the warning.

“I think Joe sometimes engages in rhetorical flourishes,” he said.

Nothing in the Constitution says the president and vice president have to get along.

In 1960, then-President Eisenhower was asked to cite an idea adopted by his administration that had come from his vice president, Richard Nixon. At the time, Nixon was running for president.

Eisenhower said, “If you give me a week, I might think of one.”

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peter.nicholas@latimes.com