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.

Former President Donald Trump is rallying against the Democratic Party Sunday, following the announcement President Joe Biden is leaving the 2024 race.

"Crooked Joe Biden is the Worst President, by far, in the History of our Nation. He has done everything possible to destroy our Country," Trump said in a statement on TruthSocial.

"Whoever the Left puts up now will just be more of the same," Trump continued.

Trump's campaign also released a statement Sunday, calling out Vice President Kamala Harris by name, after Biden endorsed her as the next Democratic pick.

"Kamala Harris is just as much of joke as Biden is," the campaign said, adding, "Harris will be even WORSE for the people of our Nation than Joe Biden. Harris has been the Enabler in Chief for Crooked Joe this entire time. They own each other’s records, and there is no distance between the two."

Democratic delegates and donors are say they are "relieved" and "thrilled" that President Biden has made the decision to step down, but they are divided on the best path forward.

"No one I know wants a coronation. In fact, Kamala will be stronger to go through a mini primary and we need to have an alternative or two," one key donor told ABC News.

"We need to put best ticket together and let delegates vote," the donor added. "Not sure it’ll happen but we are trying."

Another delegate and donor told ABC News they are fully behind Harris.

"Now big donor money will start flowing again. His selfless act will save our republic," a second donor said.

-ABC News' Selina Wang