Before Mr. Trump’s crushing victory in Pennsylvania last week, Mr. Cruz’s campaign boasted that it had 69 people devoted to acquiring as many as possible of the state’s 54 unbound delegates — who are free to vote as they please on the first ballot, making them potentially decisive players in a contested convention.

Mr. Cruz won only three.

In North Dakota, where the Cruz campaign declared victory after the state Republican convention on April 3 and declared that it had won “a vast majority” of the state’s 28 unbound delegates, Mr. Cruz’s support appears to be weakening. In interviews, delegates said he really had only about a dozen firm commitments to begin with, and some of them appear to be wavering as he falls farther behind Mr. Trump.

And in states across the South, which was supposed to be Mr. Cruz’s bulwark, some delegates are now echoing a growing sentiment inside the Republican Party: a sense of resignation to the idea that Mr. Trump will be their standard-bearer.