The mayor of Waukesha, the seat of a reliably Republican county outside Milwaukee, Wisconsin, endorsed Vice President Kamala Harris for president on Wednesday — the first time the formerly Republican city leader is supporting a Democrat for commander in chief, the campaign said.

“It would be easier for me to stay quiet and vote my conscience privately, but the stakes of this election are so important that I feel compelled to share publicly that I am voting for Vice President Kamala Harris and I encourage other Wisconsinites who care about our country to do the same,” Reilly said in a statement circulated by the Harris campaign.

Reilly added that former President Donald Trump poses “a unique danger to American democracy,” and a second term “would be even more dangerous than the first because there would be no guardrails.”

“We can’t allow him to sit in the Oval Office again,” he said.

Milwaukee’s Fox News affiliate reported the mayor’s endorsement of Harris yesterday, quoting him as saying he left the Republican Party after the Jan. 6, 2021, attack on the U.S. Capitol and voted for President Joe Biden in 2020, but still often votes Republican.

Wisconsin is a key battleground state that could be a critical piece of Harris or Trump's path to victory on election night. While the vote-rich Milwaukee suburbs haven't swung dramatically to the Democrats the way similar areas outside of big cities in battleground states have during the Trump era, the party has gradually made inroads.

Waukesha in particular has been a longtime Republican stronghold, where Democrats have made slow gains over the last two decades in statewide and federal elections.

In 2000, just 32% of voters in Waukesha County voted for Democratic presidential nominee Al Gore, while in 2020 39% of voters there supported President Joe Biden. Conversely, 65% of voters there voted for former President George Bush in 2000 and 60% of voters in Waukesha cast their vote for Trump in 2020.

In a state that Biden won by less than a point in 2020, even marginal shifts in suburban counties can influence the statewide result.

Reilly's endorsement comes as a wave of Republicans this fall have lined up behind Harris.

On Thursday, GOP former Michigan Rep. Fred Upton also endorsed Harris, saying in a statement that he has never voted for a Democrat for president "until now."

Trump is "unfit to serve as commander in chief again," Upton said.