A campaign that has played a cautious expectations-setting game is now making brash predictions, saying there is no way it will lose in Mr. Rubio’s home state. He himself left no room for error, declaring Wednesday, “We will win in Florida.”

Mr. Rubio’s biggest threat in the state, Mr. Trump, has come under an unrelenting assault from the party establishment, with the most extraordinary shot yet coming from Mitt Romney, a former Republican presidential candidate. On Thursday, Mr. Romney urged party members to rally around one of Mr. Trump’s rivals, without saying which one.

Mr. Rubio’s allies say his big bet in Florida is reflective of his tough predicament.

“Is it threading the needle?” said Tim Griffin, the lieutenant governor of Arkansas, a state Mr. Rubio lost on Tuesday. But he noted that with unexpected developments like the withdrawal of Ben Carson from the race on Wednesday, there are still a lot of unknowns. “We all talk about how difficult it is to thread a needle,” Mr. Griffin said. “But there are a lot of people walking around with clothes on. Which means there’s a lot of needles being threaded.”

The Rubio campaign and its allies insist that they have plenty of money to sustain a campaign for the foreseeable future. They have not released figures to back up their confident projections. But people who have been briefed on the fund-raising activities of the campaign and the “super PAC” backing the Florida senator say that money started rolling in after his second-place finish in the South Carolina primary. Terry Sullivan, the campaign manager, even boasted to donors last month that while his finance team might not want him to say so, they had nothing to worry about financially.

The defiance coming from Mr. Rubio and his campaign brings new attitude to a candidate always better known for his precision and predictability. Adjusting to that new role has not been without its complications.