As the calendar flips, March brings a whirlwind of states voting on the same days and in quick succession. By the middle of the month, 58 percent of the total delegates will have been awarded, and Mr. Trump could be unstoppable in getting the 1,237 needed to clinch the nomination.

With the exception of Texas, the home state of Senator Ted Cruz, recent polls show Mr. Trump leading in the so-called Super Tuesday states that vote this week, including Alabama, Georgia, Massachusetts and Virginia. Though Texas has the most delegates of states voting on Tuesday, 155, they all award delegates proportionally, so that Mr. Cruz will most likely have to share the haul.

The Southern states, with many evangelical Christians, have been the linchpin of Mr. Cruz’s strategy. But after Mr. Trump trounced him in South Carolina on Feb. 20, winning all 50 delegates, Mr. Cruz’s prospects are not as bright. Mr. Trump carries “enormous momentum,” Mr. Cruz admitted on Friday, and if he sweeps Super Tuesday, when more delegates are awarded than on any other day, “he could easily be unstoppable.”

For Senator Marco Rubio of Florida, who ferociously challenged Mr. Trump last week in a debate and on the stump, mocking the real estate mogul’s “spray tan” and calling him a “con man,” the path forward is narrowing.