COLUMBUS, Ohio (Reuters) - Democratic presidential contender Joe Biden questioned surging rival Elizabeth Warren’s credibility on Wednesday, accusing her of failing to be open with voters about how she would pay for her ambitious Medicare for All healthcare proposal.

Biden said it was “about time” that Warren, who has been steadily rising in opinion polls of the Democratic White House race for two months, face scrutiny on her plan for a government-run healthcare system.

“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,” Biden told reporters in Columbus, Ohio.

His comments came the day after a Democratic debate marked by repeated attacks on Warren by her more moderate White House rivals that exposed the party’s divisions on issues like healthcare and taxes.

Warren has risen in the past month into a virtual tie with Biden, the former vice president, atop polls to pick a challenger to Republican President Donald Trump in the November 2020 election.

In Tuesday’s debate in Ohio, moderates Pete Buttigieg, the mayor of South Bend, Indiana, and U.S. Senator Amy Klobuchar aggressively challenged Warren to explain how she would pay for ambitious proposals including the Medicare for All plan based on the government-run insurance system for Americans aged 65 and older.

Warren, a U.S. senator from Massachusetts and a leading progressive voice in the party, stayed calm under the attacks as she described her proposals to end income inequality and level the economic playing field for workers.

But Warren did not directly respond to questions about whether she would raise taxes for the healthcare plan, although she said she would not sign any bill that does not lower healthcare costs for middle-class families.

“I have made clear what my principles are here, and that is that costs will go up for the wealthy and for big corporations and, for hard-working middle-class families, costs will go down,” Warren said in the debate.

Biden said it was an ironic stance for Warren, who repeatedly promises she has “a plan for everything.”

‘NO PLAN’

“It’s fascinating that the person who has a plan for everything has no plan on the single most consequential issue in this election in the minds of the American people across the board,” Biden said.

“Tell us how you’re going to do what you say you’re going to do,” Biden said, adding “credibility matters.”

FILE PHOTO: Senator Kamala Harris, Senator Bernie Sanders, former Vice President Joe Biden, Senator Elizabeth Warren and South Bend Mayor Pete Buttigieg wait onstage before the fourth Democratic U.S. 2020 presidential election debate at Otterbein University in Westerville, Ohio October 15, 2019. REUTERS/Aaron Josefcz/File Photo

The sharp exchanges before and after the debate were a sign of the heightened stakes less than four months before the first nominating contest in Iowa on Feb. 3.

Many of the contenders also face a scramble to qualify for the next Democratic debate in Georgia in November, when polling and fundraising qualifying criteria will be even tougher. So far, just eight candidates have qualified.

Biden said it was “absolutely ridiculous” that Warren would not delve into the details of her healthcare proposal, drawing a comparison to Trump.

“The last thing the Democrats should be doing is playing Trump’s game and trying to con the American people to think this is easy. There’s nothing easy about it,” he said.

Trump has repeatedly called Biden corrupt since Democrats in Congress opened an impeachment inquiry focused on Trump’s efforts to pressure Ukraine to investigate unsubstantiated charges that Biden improperly tried to aid his son Hunter’s business interests in Ukraine.

Biden said Trump was “running the most corrupt government in the history of the United States” and taunted Trump for his refusal to release his taxes.

“Even Richard Nixon released his tax return,” Biden said. “Mr. President, release your tax returns, or shut up.”

Trump, who has blasted the impeachment probe and denied any wrongdoing, dismissed the field of Democratic candidates on Wednesday, tweeting: “Now you see why they have no choice but to push a totally illegal & absurd Impeachment of one of the most successful Presidents!”