Maryland Gov. Wes Moore endorsed Harris for the top of the Democratic ticket on Monday.

In a post on X, Moore said "Today, I am proud to voice my full support and offer my full endorsement to Vice President @KamalaHarris to be the Democratic Nominee for President of the United States."

The governor, a Democrat, issued a statement on Sunday praising Biden for his decades of service.

"President Biden has dedicated his life and career to serving the American people," Moore wrote. "His legacy of hard work, dedication, optimism, and strength have shaped the trajectory of our nation -- and made us better as a people and as a country."

-ABC News' Tommy Barone and Sarah Beth Hensley

Independent Sen. Joe Manchin, who sources had told ABC News was considering re-registering as a Democrat to run for the presidential nomination, said on Monday he is not going to be campaigning to be atop the ticket in November.

"Let me make it very clear to you, Tony: I am not going to be a candidate for president, I am a candidate for basically speaking for the middle of this country," Manchin said on "CBS This Morning."

"I am not running for office, I could not believe that there was not going to be a primary process or a mini process. I don't need that as far as in my life, the attention that people think 'oh, when you speak up you're looking for attention.' Why is everybody afraid to speak up?" Manchin said.

Instead, Manchin said he was advocating for a "process" for selecting a "new generation" of leadership.

"I think a lot of people would like to see a mini primary -- that's the process," he said.

-ABC News' Allison Pecorin

Vice President Kamala Harris, who is now frontrunner for the Democratic presidential nomination, has a good chance of making "this a very close race" against former President Donald Trump, former Gov. Chris Christie said Monday.

"Eight years ago, [Trump] ran against a woman for president. When he did, Hillary Clinton was a more known quantity and, quite frankly, was disliked by a lot of voters," Christie said on ABC News' "Good Morning America" on Monday. "Kamala Harris is not personally disliked."

He added, "There's a question of respect and whether they respect her or not. And that's going to be in her control now. If she performs well she'll make this a very close race."

A decision about Harris' vice presidential running mate will be closely watched, he said, adding that choosing Gov. Josh Shapiro could deliver his home state of Pennsylvania, which "allows them to play in Michigan, Wisconsin, Arizona."

Christie, who faced Donald Trump as a presidential candidate in the 2016 race, has become an outspoken critic of the former president. He had previously said a second Trump term would amount to a "vendetta presidency."

"And I think he will use the levers of government to punish the people who he believes have been disloyal to him or to his approach," Christie told ABC News earlier this year.

Christie was joined Monday on "GMA" by Democratic strategist and former DNC Chair Donna Brazile, who vowed the convention would have a "transparent process."

"We're going to continue to get the endorsements and, hopefully, at some point today or tomorrow the vice president will have enough delegates to secure the nomination," she said.

-ABC News' Kevin Shalvey

In the hours since President Joe Biden endorsed Vice President Kamala Harris on Sunday afternoon, she has collected $49.6 million in grassroots donations, the campaign said.

-ABC News’ Rick Klein