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

President Biden took the stage in this key battleground state intent on proving his critics wrong.

Before an amped-up crowd of 2,000 in Detroit, a fired-up President Biden railed against Donald Trump in pointed attacks, challenged the press, outlined his first 100 days in office and tried to make clear to his doubters, he isn’t going anywhere.

"Folks, I’m the nominee!" Biden roared as the crowd erupted.

"I’m the nominee as part because 14 million Democrats like you voted for me in the primaries. You made me the nominee. No one else, not the press, not the pundits, not the insiders, not donors. You the voters, you decided, no one else. And I'm not going anywhere," Biden said.

The president also framed his agenda by announcing his plan for his "first 100 days in office."

Biden pledged to codify Roe v. Wade, passing an assault weapons ban, passing the John Lewis Voting rights act and to make sure the rich pay their "fair share in taxes."

-ABC News' Mary Bruce, Molly Nagle and Will McDuffie

President Joe Biden came out strong against Donald Trump at his campaign event in Detroit, Michigan Friday.

Addressing his gaffes during the NATO conference Thursday, Biden said, "They’ve been hammering me because I sometimes confuse names. I say, that's Charlie, instead of Bill."

On Thursday, Biden referred to Ukrainian President Volodymyr Zelenskyy and Russian President Vladimir Putin.

During his press conference Thursday, Biden referred to Vice President Kamala Harris as Donald Trump.

Drawing attention to Trump’s history of name gaffes Biden said, "Donald Trump has gotten a free pass."

"I guess they don't remember that Trump called Nikki Haley Nancy Pelosi," Biden said.

"Donald, no more free passes," Biden continued, adding, "Today, we’re going to shine a spotlight on Donald Trump."