Major Democratic donor, businessman and one-time presidential candidate Michael Bloomberg is urging the Democratic Party to take a step back and "take the pulse of voters" before determining "who is best positioned to win in November," saying "the decision is too important to rush."

"Democrats now have a chance for a fresh new start, and while some elected leaders and party officials make their endorsements, there are still four weeks before the party's more than 4,000 delegates convene in Chicago," Bloomberg posted Monday on X.

"That is more than enough time for the party to take the pulse of voters, especially in battleground states, to determine who is best positioned to win in November and lead the country over the next four years," Bloomberg continued. "We don’t need to get a resolution right away, but we do need to get it right. The decision is too important to rush, because the election is too important to lose."

Bloomberg also praised President Joe Biden for stepping down, calling it "an act of selflessness that only a great patriot would do."

-ABC News' Peter Charalambous and Soorin Kim

Vice President Kamala Harris on Monday is making her first public appearance since Biden dropped out of the 2024 race and endorsed her.

Harris is giving remarks at an event celebrating the NCAA championship teams Monday morning on the South Lawn of the White House. She is filling in for Biden, who is recovering from COVID.

"I wanted to say a few words about our president. Joe Biden's legacy of accomplishment over the past three years is unmatched in modern history," Harris said at the top of her statement. "In one term, he has already surpassed the legacy of most presidents who have served two terms in office."

Harris didn’t specifically address her own candidacy, but said she was grateful for Biden's service to the country.

"I am firsthand witness that every day our president, Joe Biden, fights for American people. And we are deeply, deeply grateful for his service to our nation," she said.

-ABC News' Sarah Beth Hensley

Former Vice President Mike Pence's commented on the end of Biden's reelection bid on Monday morning, praising the president and pushing for national unity.

"President Joe Biden made the right decision for our country and I thank him for putting the interests of our Nation ahead of his own," Pence wrote in a statement on X.

"After the assassination attempt on President Trump and President Biden's decision to end his campaign, now is a time for leaders in both parties to project calm and send a message of strength and resolve to America's friends and enemies alike that, whatever the state of our politics, the American people are strong and our American military stands ready to defend our freedom and our vital national interests anywhere in the world," Pence said.

He made mo mention of Harris in the post.

-ABC News' Brittany Shepherd

Michigan Gov. Gretchen Whitmer was the final holdout of potential Biden replacements to throw her support behind the vice president.

"Today I am fired up to endorse Kamala Harris for the President of the United States," Whitmer posted to X on Monday.

-ABC News' Isabella Murray

While the vetting process is just getting started, a senior administration official told ABC News' Martha Raddatz that Pennsylvania Gov. Josh Shapiro and Arizona Sen. Mark Kelly are the leading candidates to be Harris' running mate.

The two men come from key 2024 battleground states, and have experience trouncing Trump-endorsed candidates.

Shapiro, 51, was Pennsylvania's attorney general before being elected governor in 2022 after defeating Republican Doug Mastriano.

Kelly, a 60-year-old former astronaut and Navy captain who is married to former congresswoman Gabby Giffords, won his Senate seat in 2022. He defeated Republican challenger Blake Masters.