Sen. Mitt Romney leaves the Senate floor at the Capitol on November 16, 2022 in Washington, DC.
CNN  — 

Utah Republican Sen. Mitt Romney announced Wednesday that he will not run for reelection to the Senate and called for a “new generation of leaders” as he criticized both President Joe Biden and former President Donald Trump.

In a video message, Romney outlined a range of what he described as critical issues, including rising national debt and the climate crisis, and said “neither President Biden nor former President Trump are leading their party to confront those issues.”

The Utah Republican’s decision will mark the departure of a distinctive voice from the Senate of a lawmaker willing to take on his own party. Romney, who was the 2012 GOP presidential nominee, has long been a rare member of the Republican party who has been an outspoken critic of Trump.

In 2020, Romney found Trump guilty of abuse of power during Trump’s first impeachment trial in the Senate, becoming the first senator in US history to vote to remove from office a president from the same party.

At a time when the age of prominent politicians on both sides of the aisle has come under increasing scrutiny, Romney also emphasized the need for a new generation of leadership.

“At the end of another term, I’d be in my mid-80s,” Romney, who is 76, said. “Frankly, it’s time for a new generation of leaders. They’re the ones that need to make the decisions that will shape the world they will be living in.”

“While I’m not running for reelection, I’m not retiring from the fight,” Romney said, after criticizing both Biden and Trump for failing to provide solutions to issues like the national debt and global warming. “I’ll be your United States senator until January of 2025.”

Romney heavily criticized his Senate Republican colleagues for their continued loyalty to Trump, saying, “A very large portion of my party really doesn’t believe in the Constitution,” in an excerpt published Wednesday from his upcoming biography by McKay Coppins. The excerpt, which was printed in the Atlantic, details Romney’s experience in the U.S. Senate, including his decision to not run for reelection in 2024.

Coppins recounts Romney’s experience on January 6, 2021, including the near miss where he almost walked in to a crowd of oncoming rioters, and his shock at the “disingenuousness” of his colleagues that continued to object during the electoral college proceedings. “‘They know better! Josh Hawley is one of the smartest people in the Senate, if not the smartest, and Ted Cruz could give him a run for his money,’” Romney told Coppins. As for his newer colleagues, Romney said, “’I don’t know that I can disrespect someone more than J.D. Vance.’”

Coppins says Romney also told him that other Republican senators that ended up voting to acquit Trump in his second impeachment trial changed their minds out of fear for their safety and the safety of their families.

Romney also acknowledged his concerns about Biden’s age in the next election, telling Coppins, “I was afraid that Biden, in his advanced years, would be incapable of making the argument.” He said he had a pipe dream of running again, just to push back on Trump’s rhetoric, and had even approached Democratic Sen. Joe Manchin of West Virginia about forming a third party.

Romney has had a prominent career on the national stage in American politics in addition to his time spent serving in the Senate. He ran as the Republican Party nominee in the 2012 presidential race and lost to Barack Obama, who was then the Democratic incumbent. Romney also served as the governor of the state of Massachusetts from 2003 to 2007.

In making his announcement, Romney outlined the issues he sees with both Trump and Biden.

“On deficits and debt, both men refuse to address entitlements even though they know that this represents two-thirds of federal spending. Donald Trump calls global warming a hoax, and President Biden offers feel-good solutions that make no difference to the global climate. On China, President Biden underinvests in the military and President Trump underinvests in our alliances,” he said.

Senate Republican leader Mitch McConnell praised Romney in a statement reacting to the news.

“The US Senate is known to attract bright and proven public servants. However, we rarely get to welcome new Senators already as accomplished and well-regarded as Mitt Romney. The Senate has been fortunate to call our friend from Utah a colleague these past four and a half years, and I am sorry to learn that he will depart our ranks at the end of next year,” McConnell said, adding, “I know the Senate is relieved that we will continue to benefit from Senator Romney’s expertise throughout the coming year.”

This story has been updated with additional details.

CNN’s Jeremy Herb contributed to this report.