Sixty-three signatories had joined onto the letter, which was obtained by ABC News, as of early Sunday evening, but more signatures could be added.

"We are deeply grateful to President Biden and his family for their decades of service to our nation, including the decision he made today to pass the torch of the Democratic nomination and American democracy itself to a new generation of public servants. We strongly and enthusiastically endorse Vice President Kamala Harris -- the candidate with the best experience and capacity to unite our Party and the United States -- for the Democratic nomination for President of the United States. We ask that you join us," they wrote.

The International Association of Fire Fighters, which was the first union to back Joe Biden in the 2020 presidential election, released a statement on his decision to exit the 2024 race.

"Past, present, and future fire fighters are better off because of Joe Biden," IAFF general president Edward A. Kelly said Sunday.

"For nearly 50 years, he has been a champion of the IAFF. His work has directly led to the hiring of tens of thousands of fire fighters, safety improvements in this most dangerous of jobs, and a nation that is now aware of the toxic threats we face every time we put on our gear," Kelly said.

"And, when a fire fighter makes the ultimate sacrifice, President Biden’s work has ensured their families receive the benefits they deserve," Kelly continued.

Kelly called Biden the "greatest President fire fighters and emergency medical workers have ever had."

"Our membership – and the communities we serve – will forever be in his debt," Kelly said.

Former Vice President Al Gore joined the chorus of Democratic leaders weighing in on President Biden's decision to leave the 2024 presidential race on Sunday.

Praising Biden's significant action toward climate change solutions, Gore said in a statement on X, "As President, Joe Biden has been — and is — an exceptional leader. He is responsible for developing and signing the most significant investment in climate solutions into law."

"I know that he is not done delivering for the American people and will continue to serve this nation with wisdom and dignity in the coming months," Gore continued, adding, "In paving the way for a new generation of leadership, President Biden’s decision is a fitting testament to his profoundly consequential career."

"I applaud his selfless decision," Gore wrote.

5:55

Harris says she's ‘honored to have the president’s endorsement’

ABC News’ Mary Bruce and Jon Karl discuss the statement released by Vice President Kamala Harris.

In a statement announcing her candidacy for president, Vice President Kamala Harris thanked President Joe Biden for his "extraordinary leadership."

"I thank Joe Biden for his extraordinary leadership as President of the United States and for his decades of service to our country. His remarkable legacy of accomplishment is unmatched in modern American history, surpassing the legacy of many Presidents who have served two terms in office," Harris said.

"With this selfless and patriotic act, President Biden is doing what he has done throughout his life of service: putting the American people and our country above everything else," Harris said.

Biden endorsed Harris after announcing his exit on Sunday, saying choosing her as his running mate in 2020 was the "best decision I've ever made."

"Over the past year, I have traveled across the country, talking with Americans about the clear choice in this momentous election. And that is what I will continue to do in the days and weeks ahead. I will do everything in my power to unite the Democratic Party—and unite our nation—to defeat Donald Trump and his extreme Project 2025 agenda," Harris said.