One of President Biden's closest allies, Michigan Rep. Debbie Dingell, mourned the end of Biden's presidential campaign while endorsing Vice President Kamala Harris on Sunday.

"I did endorse Kamala this afternoon because I believe Joe Biden has been a great President," Dingell said during an interview on MSNBC.

"I think it is a sad day and the way much of this has played out with the knives that have been put in his back," she said of Biden.

"If he is supporting Kamala Harris, that means that he thinks that she can do a good job and that is who I am supporting," Dingell continued, adding, "This party needs to unify."

Upon learning President Joe Biden would end his race for a second term, White House staff members told ABC News how they’re feeling about the historic decision.

Staffers said emotions have been running high, and that -- blindsided by the announcement -- some had cried. One said he had been on his bike and nearly fell off when he heard the news.

Some said they felt shocked or heartbroken, but many expressed their relief.

"He made the right choice; acted in the best interest of the country,” one White House staff member said.

Others said they felt immense pride. "I will always be proud that I worked for him, and I think history will show him to be one of the greats,” one staffer said.

"Any Democratic president in the past 50-70 years would kill for some of the big wins Joe Biden has [gotten] across the finish line,” another said. “He is a good man and fierce defender of democracy."

-ABC News' MaryAlice Parks

The Biden for President campaign committee has just been changed to the Harris for President campaign committee, according to a new FEC filing.

Now Kamala Harris' principal campaign committee, this campaign committee had nearly $96 million in cash on hand -- and likely more scheduled to be transferred from its joint fundraising committees with the DNC.

-ABC News' Soorin Kim

Larry Hogan, the former Maryland governor who is running for U.S. Senate, released a statement on X Sunday after President Biden ended his bid for reelection.

"I believe he made the right decision for his family and the country," Hogan, a Republican, said of Biden, adding, "These are dangerous and unprecedented times. He should be commended for putting the country ahead of personal ambition."

"The only way out of this cycle of chaos, divisiveness and dysfunction is to elect principled, common sense and experienced leaders who will focus on the day-to-day challenges facing Americans," Hogan continued.

"We need less palace intrigue, less partisan nonsense, and more decisive action to solve the serious problems facing our nation," Hogan said.