Sen. Amy Klobuchar, D-Minn., told ABC News she is backing Vice President Kamala Harris for president in the 2024 election.

"I worked with her in the Senate for years and we became closer friends even when we ran against each other," Klobuchar said.

Klobuchar said President Joe Biden chose "the honorable path" by dropping out.

"President Biden has served with integrity and delivered results that will strengthen our country for generations to come. He passed historic legislation investing in our infrastructure, manufacturing and lowering the cost of prescription drugs. I am deeply grateful for his service, dedication, and commitment to our country and our democracy," Klobuchar said.

-ABC News' Selina Wang

Marianne Williamson is throwing her hat back in the 2024 presidential election ring after President Joe Biden exited the race Sunday.

"The nomination of a new Democratic candidate must be opened to a genuinely democratic process at an open convention," Williamson told ABC News in a statement.

"No one should simply be anointed to the position of nominee; all candidates must be heard and their agendas explored," Williamson said, seemingly referencing Biden's endorsement of Vice President Kamala Harris.

"Our party’s basic first principle is democracy," Williamson continued, adding, "We cannot save our democracy without practicing it ourselves."

"I look forward to taking my message to the American people, and convincing Democratic delegates, that I am the best candidate to take us to victory in November," Williamson said.

In a statement, former President Barack Obama lauded President Joe Biden for his decades of service and decision to leave the race. He did not mention Vice President Kamala Harris in the statement.

"Joe Biden has been one of America’s most consequential presidents, as well as a dear friend and partner to me," Obama wrote. "Today, we’ve also been reminded -- again -- that he’s a patriot of the highest order."

Obama praised Biden’s "outstanding track record" in office and how he "has never backed down from a fight."

"For him to look at the political landscape and decide that he should pass the torch to a new nominee is surely one of the toughest in his life. But I know he wouldn’t make this decision unless he believed it was right for America," he wrote. "It’s a testament to Joe Biden’s love of country -- and a historic example of a genuine public servant once again putting the interests of the American people ahead of his own that future generations of leaders will do well to follow."

Obama said he has "extraordinary confidence that the leaders of our party will be able to create a process from which an outstanding nominee emerges."

"For now, Michelle and I just want to express our love and gratitude to Joe and Jill for leading us so ably and courageously during these perilous times --  and for their commitment to the ideals of freedom and equality that this country was founded on," he wrote.

Ahead of President Joe Biden's announcement that he dropped out of the 2024 presidential election, Sen. Elizabeth Warren, D-Mass., had voiced her support for Vice President Kamala Harris.

"If President Biden decides to step back, we have Vice President Kamala Harris, who is ready to step up, to unite the party, to take on Donald Trump, and to win in November," Warren said.