ROLL CALL: Marco Rubio’s Presidential Bid Creates Open-Seat Scrum.

Marco Rubio on Monday became the third Republican senator to announce a bid for president and the first of the group to leave behind a competitive seat.

Rubio vacating his seat in Florida in favor of national ambition gives the GOP another potentially strong White House contender. But it presents new challenges for Senate Republicans pushing to hold their newly acquired majority by leaving an expensive open seat in a state at the heart of the presidential election.

Two days before Rubio’s formal Monday night announcement in Miami, the race to replace him had already taken an unforeseen turn. Republicans thought they had a top recruit in Florida Chief Financial Officer Jeff Atwater, until he announced on April 11 he wouldn’t run. His decision leaves a wide-open field on the Republican side, with at least a half-dozen potential contenders.

“The race has changed dramatically,” said Adam Goodman, a Republican media consultant in Florida. “There was an emerging consensus that Atwater was certainly, if nothing else, going to be given the first full shot to close out the field and move ahead with the nomination. So everyone had to wake up to the realization [on Saturday] that suddenly the world has changed.”

Since Atwater’s exit, some Republicans have turned their sights to the congressional delegation, particularly Tom Rooney, a four-term Republican from central Florida.

Rooney, who told CQ Roll Call two weeks ago the race wasn’t even on his radar, said in a Monday phone interview he now believes he can win the open seat and has a meeting scheduled at the National Republican Senatorial Committee Thursday.

A potential campaign, he said, is “something that I’m taking very seriously, and I’m talking to a lot of people.” Rooney, the lone member from Florida so far to endorse Rubio, noted he has lingering concerns on the effect a statewide race will have on his wife and three sons, ages 13, 11 and 8.

Republicans are also looking at former state Speaker Will Weatherford, seen as a possible Rubio replacement for some time. The senator even mentioned Weatherford as a potential successor at a breakfast with reporters in January. But Weatherford hadn’t expressed much interest when Atwater was a seemingly certain contender, and Florida Republicans wonder if he would prefer to hold off.

Stay tuned, I guess.