A former North Carolina governor, Roy Cooper, broke fundraising records in his first 24 hours as a Senate candidate in the Tar Heel State, signaling strong momentum for what is being predicted to be one of the most expensive statewide races in American history.

With $3.4 million raised within the first day after staking his claim for the seat after the incumbent, Senator Tillis, decided not to run for re-election, the race pitting Mr. Cooper against the Republican National Committee chairman, Michael Whatley, is shaping up to be one of the costliest in next year’s midterm elections, with estimates as high as $800 million.

The previous record of nearly $500 million was set in 2022 during the Georgia Senate race in which the incumbent Democrat, Senator Warnock, defeated the Republican candidate, Herschel Walker, a former professional football player.

“You have a two-time governor, now seeking the U.S. Senate, running against, essentially, the RNC, so both candidates, both parties, are going to bring a lot of money to this race,” a Democratic state senator, Jay Chaudhuri, said to Politico.

“Not since Jesse Helms versus Jim Hunt” in the 1984 North Carolina Senate race “have you had two such national prominent candidates run against each other.”

Mr. Tillis announced last month that he would not be seeking re-election in 2026, just hours after President Trump threatened to throw his weight behind a Republican primary challenger after the senator voted against a motion to proceed to debate the president’s One Big Beautiful Bill Act in the Senate.

“North Carolina will not allow one of their Senators to GRANDSTAND in order to get some publicity for himself,” Mr. Trump wrote on Truth Social after Mr. Tillis voted against the procedural motion.

Mr. Tillis said in his announcement that while he would not be seeking re-election, it was a “blessing” to be able to represent his state but he was not looking forward to six more years of “political theatre” and “partisan gridlock.”

 “It’s not a hard choice, and I will not be seeking re-election,” he said.

Republicans believe that Mr. Cooper’s middle-of-the-road demeanor won’t translate into a successful campaign.

Mr. Tillis has weighed in on Mr. Cooper’s announcement and the upcoming race, acknowledging when asked by reporters on Monday night that the new candidate has a “pretty good track record as governor,” but gave a caveat, saying he is “not invulnerable.”

“We’ve just got to match up and beat him,” he said. “I don’t want to see this seat be taken by a Democrat.”