The Florida senator and his advisers have concluded that a head-to-head battle with Mr. Trump over the next several weeks would be much more advantageous than one with Senator Ted Cruz of Texas, whose success would greatly complicate Mr. Rubio’s hopes of consolidating his support inside the Republican Party.

A victory by Mr. Trump would send panicked Republicans toward Mr. Rubio, his campaign reasons, especially donors who have been reluctant to get behind Mr. Rubio because their allegiances are with other candidates.

A victory by Mr. Cruz, however, would give the Texas senator a lift in states that hold later primaries and caucuses, and would draw voters away from Mr. Rubio. And with internal polling showing significant overlap with Cruz voters, the sooner Mr. Cruz can be hobbled, the better, Rubio advisers say…

The Rubio campaign sees Trump voters as essentially “immovable granite,” in the words of one adviser: They are unlikely to be swayed by attacks on Mr. Trump and they would probably never vote for Mr. Rubio.