Former GOP Sen. Jeff Flake endorsed Vice President Kamala Harris for president on Sunday, writing in a statement that he wanted to support a presidential candidate who represents “a new generation of leadership based not on grievances of the past, but hope for the future.”

“I’ve served with Kamala in the U.S. Senate. I’ve also served with [Minnesota Gov. Tim Walz] in the House of Representatives. I know them. I know first hand of their fine character and love of country,” the former senator from Arizona added in his statement posted on X.

Flake previously endorsed Joe Biden for president in 2020 and served as the ambassador to Turkey under Biden before stepping down earlier this month.

The Republican mentioned that experience in his endorsement, writing that having spent “the past three years overseas as a U.S. Ambassador, I’ve seen close up that we have very real enemies abroad. We also have vital and indispensable allies. I want to support a president who understands and appreciates the difference.”

Even before the 2020 presidential election, Flake had long sparred with Donald Trump.

In 2016, he was a vocal critic of the then-Republican presidential nominee and said he would write in a candidate rather than vote for former Secretary of State Hillary Clinton or Trump.

After he announced in 2017 that he wouldn’t seek re-election to the Senate the following year, Flake continued to be outspoken in his opposition to Trump, saying at the time that “there may not be a place for a Republican like me in the current Republican climate or the current Republican Party.”

His endorsement of Harris comes as her campaign has for months reached out to prominent anti-Trump Republicans seeking endorsements.

In August, the campaign unveiled more than two dozen Republican endorsements for the vice president. And in September, former GOP Rep. Liz Cheney, another high-profile anti-Trump Republican, endorsed Harris. Days later, her father, former Vice President Dick Cheney, also said he would vote for Harris.

The Democratic National Convention in August also featured prominent Republican speakers backing Harris and Walz, including former Georgia Lieutenant Governor Geoff Duncan, former Illinois Rep. Adam Kinzinger and former Trump White House Press Secretary Stephanie Grisham.

In a statement, Austin Weatherford, the Harris campaign's National Republican Outreach Director, highlighted the campaign's Republicans for Harris program, which seeks to deploy the Republicans who are backing Harris to battleground states to convince other traditionally GOP voters to back Harris at the polls.

"Our Republicans for Harris program is taking that unifying, inspiring message to anti-Trump Republicans, moderates, and independents. While we’re seeing a surge in support, we aren’t taking anyone for granted,” Weatherford said.

Later Sunday, Flake spoke with ABC News' "This Week," saying that many conservative Republicans feel the same way about the former president.

"I think Republicans believe in the rule of law in particular," he said. "It's difficult to support a candidate who, having lost an election, tries to use the powers of the presidency to overturn that election. That is anything but respect for the rule of law. So I know that a lot of conservative Republicans feel the way I do, that you just can't support a candidate like that."