Rubio’s renewed Florida focus comes as party leaders, donors and other supporters are worrying about his political operation, which they say continues to come up short in its message, in its attention to the fundamentals of campaigning and in its marketing of a promising politician. The failures have all but doomed ­Rubio’s chances of securing the GOP nomination, they say, leaving him far behind Trump and Cruz in both delegates and states won.

One prominent Rubio supporter, an elected official and spoke on the condition of anonymity to comment candidly, told The Washington Post that he now doubts that Rubio can win and is privately preparing to support Cruz should the race narrow. He said Rubio’s recent decision to go hard in attacking Trump does not seem to have helped him.

Rubio batted away those criticisms Monday, saying that “every campaign” is stretched thin as a presidential campaign continues.