COLUMBUS, Ohio — A day after their fiercest debate-stage confrontation to date, Joseph R. Biden Jr. escalated his criticism of his most formidable rival for the Democratic presidential nomination, Senator Elizabeth Warren of Massachusetts, suggesting that she was not being forthright about the costs of her far-reaching policy proposals and questioning her “credibility.”
Mr. Biden zeroed in on Ms. Warren’s refusal to say if she would raise taxes on middle-class Americans to pay for “Medicare for all,” a single-payer health care system that would all but eliminate private health insurance. While other candidates are pouncing on that issue as well, Mr. Biden is under particular pressure because Ms. Warren has passed him in some national and early-state polls and joined him as a front-runner, a status he held alone for months.
“It’s fascinating that the person who has a plan for everything has no plan for the single most consequential issue in this election in the minds of the American people across the board,” Mr. Biden said, addressing reporters on a cold, gray afternoon outside a union electrical trade center.
“And, you know, credibility matters. It matters. And the question that I think that Senator Warren is going to have to face, she’s going to have to tell the truth or the question will be raised about whether or not she’s going to be candid and honest with the American people,” he said.
He noted that she supports several ambitious proposals, including a tax on the wealth of the richest Americans, and said she should be pressed for details on how to implement them.
“The last thing the Democrats should be doing is playing Trump’s game, trying to con the American people to think this is easy,” Mr. Biden said.
Ms. Warren’s team did not respond directly to Mr. Biden’s attacks on policy matters, instead skewering his campaign’s reliance on high-dollar fund-raising events.
“Instead of speaking to wealthy donors, our campaign spent the day calling and thanking over a thousand of our grass-roots supporters,” said Kristen Orthman, the communications director for Ms. Warren’s campaign. “Other than that we have no comment.”
Mr. Biden and his allies have been previewing attacks on Ms. Warren for more than a month, swiping at her affection for sweeping policy plans and casting them as unrealistic or polarizing. But at last month’s presidential debate, the two barely clashed.
That dynamic changed dramatically on Tuesday, when Mr. Biden accused Ms. Warren of “being vague” in her proposals. Ms. Warren went on to detail her work in helping to create the Consumer Financial Protection Bureau under the Obama administration, leading to a tense exchange with Mr. Biden, who was vice president at the time.
“I went out on the floor and got you votes,” Mr. Biden said, his voice rising and hand pointing in Ms. Warren’s direction. “So let’s get those things straight, too.”
Mr. Biden is now confronting a series of challenges: a third-quarter fund-raising haul that fell well behind the totals of Ms. Warren and Senator Bernie Sanders of Vermont; struggles in some early state polls; and scrutiny on his family after revelations that President Trump asked the Ukrainian government to investigate Mr. Biden and his son Hunter.
There is no evidence that Mr. Biden, as vice president, acted improperly to aid his son’s overseas business dealings, but the development has emerged as a major distraction to his campaign, though his national polling numbers have generally remained steady since the Ukraine controversy broke last month.
On Wednesday, Mr. Biden again sought to put that story line behind him, repeatedly working to bring the focus back to Mr. Trump and casting him as dangerous and corrupt.
“Mr. President, release your tax returns or shut up,” he said at one point, looking directly into the assembled cameras.
And Mr. Biden, who often talks about civility, appeared far more eager to seize opportunities to tweak his Democratic rivals than he has on many previous occasions.
“God love Bernie, you know it was really good, I really mean it, to see him so healthy and moving last night,” Mr. Biden said of Mr. Sanders, who recently suffered a heart attack. “But Bernie! Bernie doesn’t pay for half his plan.”
Asked about Ms. Warren’s remark at the debate, that the United States should “get out of the Middle East,” Mr. Biden, a former chairman of the Senate Foreign Relations Committee, embraced the chance to attempt to draw another contrast.
“I quite frankly was surprised,” he said. “I have never heard anyone say, with any serious background in foreign policy, that we pull all troops out of the Middle East.”
Ms. Warren’s campaign has clarified that she was referring to combat troops.
Later Wednesday, Mr. Biden gave a foreign policy address in Iowa where he briefly referenced that moment, again noting his surprise at her remarks before turning back to his foreign policy agenda.
To some extent, Mr. Biden’s attack is part of a broader effort to cast himself as uniquely qualified on foreign policy, an argument he made explicitly in his speech in Iowa.
“Trump is demolishing the moral authority of the United States of America,” he said, and the next president will have to handle foreign relations in “whatever the state of disarray Trump leaves it in.”
“We need a leader who can on Day 1 pick up the phone, call our NATO allies, know them by their first names, and know and have them know there will be no question about the word of the next president of the United States,” Mr. Biden said. “I’ve personally met and built relationships with almost every world leader that’s still on the stage.”
Thomas Kaplan and Maggie Astor contributed reporting from New York.