Party leaders, donors and other supporters of Rubio portray a political operation that continues to come up short in its message, in its attention to the fundamentals of campaigning and in its use of a promising politician. The failures have all but doomed ­Rubio’s chances of securing the GOP nomination, leaving him far behind Trump and Cruz in both delegates and states won…

“They have no infrastructure,” said Scott Reed, who is unaffiliated with any campaign but serves as the chief political strategist for the U.S. Chamber of Commerce. “His campaign hasn’t been able to keep up with his candidacy. . . . They don’t have the operation in the states to help him get over the top. He should be a finalist going all the way to California, and he’s not.”

One prominent Rubio supporter, who is an elected official and spoke on the condition of anonymity to comment candidly, said Sunday 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.

“Cruz seems to be the only one benefiting from Marco’s hits on Trump. Marco hits Trump and doesn’t go up in the polls,” the supporter said. “It wasn’t easy to watch television on Saturday night. Nothing is happening. A lot of us, we love him, but we think we might have to end up with Cruz.”