Since Thanksgiving there has been a gradual shift among prominent Democrats once deeply skeptical of Biden’s candidacy. In national polls, Elizabeth Warren, who was on a trajectory to topple Biden, has lost all the gains she achieved since July and fallen to third place. Pete Buttigieg, who has replaced Warren as the hot candidate among white college-educated voters, has shown no evidence that, even as he thrills a subset of the Democratic electorate in Iowa, he can achieve broad appeal among African American and Latino voters. Meanwhile, Sen. Bernie Sanders, who won 43 percent of the primary vote in 2016, has been unable to break out of the mid-teens for most of this year.

Biden’s base of older working-class white and African American voters has been unassailable. A year of national polling of the Democratic primary shows his remarkably consistent support. According to the Real Clear Politics national polling average on Dec. 8, 2018, Biden had 29 percent support nationally. On Dec. 8, 2019, he had 29 percent support.

The greatest threats to Biden’s African American base have been neutralized. Sen. Kamala Harris has suspended her campaign. Sen. Cory Booker has struggled to qualify for the PBS NewsHour/POLITICO Debate, on Dec. 19. Former Gov. Deval Patrick, a pre-Thanksgiving entrant to the race, has barely been heard from and is polling at less than 1 percent.

This current bright spot could just be some momentary sunshine for Biden before storm clouds gather. Biden’s position in Iowa and New Hampshire, overwhelmingly white states packed with college-educated Democrats unexcited by his candidacy, is middling. Poor showings in both states could upend the race. Warren, who has built a robust national campaign and is generally seen by Biden’s advisers as the strongest primary opponent remaining, could still make a comeback. Biden has struggled with fundraising, while all three of his top opponents — Warren, Sanders, and Buttigieg — have demonstrated the potential to fund a long campaign against him. Mike Bloomberg, the billionaire who has quietly bought himself into fifth place by spending tens of millions of dollars on television ads, looms as a potential Super Tuesday threat on March 3, after the first four states have voted.

But, for now, Biden remains the favorite, a fact that even the online betting markets, which were wildly bullish on Warren from September through November, now acknowledge.

And so the question of how to address Biden’s age, which may be the candidate’s most significant liability, and the related question of the lack of enthusiasm for Biden’s candidacy among the activist wing of their party, has once again seized Democrats.

A top Biden adviser said Biden ruled out a one-term pledge when the issue was raised before he even entered the race. “He said it was a nonstarter,” the adviser said, adding that Biden believed it was a “gimmick.”

But Biden’s public statements on running for reelection have shifted over the course of the campaign.

In April, when asked whether he would serve just one term, Biden responded, “No.” More recently, Biden has been ambiguous. In October, The Associated Press reported that when “asked whether he would pledge to only serve one term if elected, Biden said he wouldn’t make such a promise but noted he wasn’t necessarily committed to seeking a second term if elected in 2020.”