Allies of Joseph R. Biden Jr., concerned about his slipping poll numbers in the Democratic presidential primary and an onslaught of attacks from President Trump, are weighing whether to mobilize a super PAC supporting Mr. Biden and have held conversations with wealthy donors to gauge their interest in contributing money.
Mr. Biden and his campaign aides have said since the start of the race that they would not welcome an outside spending effort on his behalf, and every other major Democratic candidate has sworn off super PACs for the duration of the primary. A spokesman for Mr. Biden on Thursday reiterated that he opposed super PACs and would reject any such group that attempted to support him.
Yet several former staffers of Mr. Biden and political donors backing his candidacy have held conversations in recent weeks about moving ahead with a super PAC, and said Mr. Trump’s furious, and often unsubstantiated, allegations about the former vice president had convinced them it was imperative they act.
“For me, this week puts everything into stark relief,” said Larry Rasky, a former aide to Mr. Biden who is now a political fund-raiser and public relations executive. “And I was already thinking the campaign was being a little naïve about the resources we’d need to fight this.”
In Mr. Rasky’s office in Washington last week, a small group of Biden allies briefly discussed the possibility of a pro-Biden super PAC. Philip Munger, a prominent Democratic donor, and Mark Riddle, a political strategist, were among those who participated.
Conversations about a super PAC were well underway earlier this month, even before news broke of Mr. Trump’s attempt to enlist the leader of Ukraine to investigate Mr. Biden. Mr. Riddle was involved in filing paperwork for a pro-Biden super PAC, named For the People, earlier this year. The Biden campaign disavowed it last spring, and there is no evidence that the group has raised any money since then.
Mr. Riddle suggested that could change.
“The vice president is getting hit from all directions,” Mr. Riddle said in an interview, alluding to some negative advertising by a Republican group, Great America PAC. “A lot of us believe there should be a fair fight. He can hold his own with his campaign in these early states or whatnot, but there’s only so much incoming somebody can take without a response.”
T.J. Ducklo, a spokesman for Mr. Biden, said Mr. Biden would reject any super PACs that joined the race on his side. Invoking the Supreme Court decision that made such groups legal, Mr. Ducklo said Mr. Biden “has long advocated to reverse Citizens United and stand up against the corrupt money in our politics.”
And while some of Mr. Biden’s supporters see the anti-Biden efforts by Mr. Trump’s allies as a reason to solicit their own outside money, Mr. Ducklo said the campaign took an opposing view.
“The attacks aimed at this campaign from dark money groups helping Donald Trump spread his outlandish lies and slander have only served as a reminder of the urgent need for campaign finance reform,” Mr. Ducklo said. “Which is exactly why since the beginning of this campaign, Biden for President has not and will not welcome the help of super PACs. That goes for those that purport to help him, despite his explicit condemnation of their existence.”
In addition to its public statements, Mr. Biden’s campaign has warned off some Democrats from participating in a super PAC. One prominent Democrat, David Wilhelm, a former chairman of the Democratic National Committee, said he had been approached about being involved in the pro-Biden super PAC but had declined to do so after the campaign urged him against it.
Mr. Riddle said a final decision about whether to deploy a super PAC for Mr. Biden would be made in “the coming weeks.”
Mr. Biden clearly needs to jolt his campaign, which has lost ground in the early nominating states to Senator Elizabeth Warren, who has been largely unscathed by her rivals. And the vice president’s longtime friends increasingly believe that Ms. Warren’s growing strength — along with Mr. Trump’s offensive — is reason enough to risk the backlash they would get from their opponents and campaign finance reform advocates for creating a group reliant on large contributions. But Mr. Riddle said there had been no final decision on whether to proceed with a super PAC, in part because of this risk. Mr. Biden’s two leading competitors for his party’s nomination, Ms. Warren and Senator Bernie Sanders of Vermont, have made getting big money out of politics a major theme of their campaigns. And Mr. Biden has already drawn criticism for his comparative reliance on wealthy donors to fund his candidacy.
“I’m mindful that any effort to do this is going to be disowned by the campaign because of the way Warren has replayed her hand,” said Mr. Rasky, alluding to the senator’s decision not to solicit large campaign checks as a presidential candidate, despite having done so in her Senate campaign.
Mr. Riddle said that any pro-Biden advertising would be focused on responding to attacks from Mr. Trump and Republicans, rather than targeting other Democrats.
Despite Mr. Munger’s presence at the meeting last week, Mr. Riddle said Mr. Munger was not involved in devising the pro-Biden group and had not contributed any money so far. Mr. Munger is the son of Charles Munger, a billionaire who is the vice chairman of Berkshire Hathaway, Warren Buffett’s investment firm.
There have been a few fitful attempts so far to use super PACs on behalf of specific Democratic presidential candidates, but none of them has had a major impact on the race. A super PAC promoting Gov. Jay Inslee of Washington ran a wave of television ads in Iowa, but Mr. Inslee did not rise in the polls. And Senator Cory Booker of New Jersey sternly disavowed an effort by one Democratic donor, Steve Phillips, to lead a super PAC promoting his campaign.
Tiffany Muller, president of End Citizens United, a progressive group that supports stricter campaign finance laws, said Mr. Biden was right to reject any group that might attempt to raise and spend money in unlimited increments. She credited him with being “an outspoken opponent of single-candidate super PACs.”
“The Democratic nominee should be chosen by voters, not millionaires and corporate special interests,” Ms. Muller said.
Still, Mr. Biden’s political position now appears precarious enough that some of his supporters believe it may be worth taking the considerable risk of activating a group that could raise and spend large sums of money to support him.
While Mr. Biden remains at the top of the Democratic field, his poll numbers have slowly weakened over the course of the race, leaving him at risk of slipping behind Ms. Warren or Mr. Sanders in three of the four early primary and caucus states. A poll released this week by Quinnipiac University found Ms. Warren slightly ahead of Mr. Biden in the preferences of Democratic voters nationwide, though her lead was less than the margin of error.
The next round of fund-raising reports, due after the end of the month, will also reveal whether Mr. Biden is still in a strong financial position relative to his nearest competitors. In the last quarter, Mr. Biden raised the second-most money of any candidate, trailing only Pete Buttigieg, the mayor of South Bend, Ind. But he finished June with less money in the bank than Mr. Buttigieg, Ms. Warren, Mr. Sanders and Senator Kamala Harris of California.