A call between Biden and the New Democrat Coalition, a group of center-left congressional Democrats, got heated, a source with knowledge of the conversation told ABC News.

Biden got angry with the group, yelled at one, Rep. Jason Crow of Colorado, and then got off the call before other members could confront him about stepping aside, according to the source.

"It was bad," one member on the call said, according to the source. "No plan."

When members pressed him on how he would turn the campaign around, Biden just listed policy achievements and insisted his polls look better, the source said.

Biden got off the call before Rep. Mike Quigley, who has called on him to step aside from his campaign, could ask a question, according to the source.

-ABC News' Rachel Bade

Biden spoke with the Congressional Progressive Caucus members Saturday to discuss his plan moving forward with his campaign.

Washington Rep. Pramila Jayapal, the caucus' chair, said in a statement that the conversation was "productive and engaging."

"We spoke frankly to the President about our concerns and asked tough questions about the path forward. We appreciate his willingness to thoughtfully answer and address our Members," she said in her statement.

Saturday's meeting came a day after the president met with the Congressional Asian Pacific American Caucus and the Congressional Hispanic Caucus's political arm, BOLD PAC, Friday.

-ABC News' Fritz Farrow and Lauren Peller

Vermont Sen. Bernie Sanders issued a firm backing for Biden's re-election bid in a guest essay published in the New York Times Saturday.

Sanders, who previously challenged Biden in the 2020 primary, acknowledged his differences with the president on several issues and concerns over Biden's age. However, he chastised Democrats who have come out calling for Biden to step aside.

"Enough! Mr. Biden may not be the ideal candidate, but he will be the candidate and should be the candidate. And with an effective campaign that speaks to the needs of working families, he will not only defeat Mr. Trump but beat him badly. It’s time for Democrats to stop the bickering and nit-picking."

Among the Democrats who have called for Biden to end his bid is fellow Vermont Sen. Peter Welch.

Sanders did not directly mention Welch, who is currently the only senator to openly call on Biden to bow out, in the essay.

Instead, he focused on the dangers of Trump's policies and rhetoric and played up Biden's accomplishments in several areas including income inequality.

"This election offers a stark choice on issue after issue. If Mr. Biden and his supporters focus on these issues — and refuse to be divided and distracted — the president will rally working families to his side in the industrial Midwest swing states and elsewhere and win the November election. And let me say this as emphatically as I can: For the sake of our kids and future generations, he must win," Sanders said.

Rep. Haley Stevens, a Michigan Democrat, said after President Biden’s Detroit rally that Biden "is the only candidate with a proven ability to beat Donald Trump."

Amid concerns from some who have witnessed the president behind closed doors about his ability to serve four more years, Stevens’ pledge of support comes after she spent the morning with the president on Air Force One, where, she said, "he demonstrated his leadership and wisdom, yet again."

"Now is the time to stay the course, work hard, ensure we beat Donald Trump, and protect our democracy for generations to come," she said.

Fellow Michigan Democrat Rep. Shri Thanedar, who traveled with Biden during the visit, told ABC News the president "delivered one of his strongest speeches."

"The more people try to pull him down - the stronger he's gonna get. It's energizing him - because he's a fighter," he said.

-ABC News’ Will McDuffie and Zohreen Shah