“Donald Trump was uncharacteristically low energy,” Mitt Romney, the Republican nominee in 2012, said in an interview Sunday on NBC’s “Meet the Press,” taunting Mr. Trump with the insult Mr. Trump had employed against Jeb Bush. Yet despite the renewed optimism of his opponents, the path to deny Mr. Trump the nomination remains narrow and arduous.

Mr. Cruz’s emergence as the most credible alternative to Mr. Trump has proved both a boost and a complication for those seeking to derail the New Yorker. Mr. Cruz has tried to undercut calls for a contested convention to deny Mr. Trump the nomination, which Mr. Cruz says would yield a “manifest revolt” among voters. But Mr. Cruz has done little so far to threaten Mr. Trump’s lead in the delegate race.

Much of Mr. Cruz’s late-breaking support on Saturday seemed to come at the expense of Mr. Rubio, not Mr. Trump. And the Cruz campaign’s message of ideological purity and religious faith is a less natural fit for many of the delegate-rich Midwestern and coastal states that remain on the map.

“Saturday proved that Trump can be contained and even beaten,” said Scott Jennings, a longtime Republican strategist, who looked ahead to this summer’s Republican convention in Cleveland. “The question is whether the field is going to allow for it moving forward. The most likely scenarios remain that Trump gets enough before Cleveland, or nobody does. The latter moved a little closer to realistic Saturday.”

Mr. Rubio’s path is much less certain, despite his lopsided victory in Puerto Rico on Sunday. Even his supporters said that the results on Saturday seriously undercut the premise of his bid: that he is the only candidate who can unify the Republican Party and defeat Mr. Trump.

“Look, I’m supportive of Marco; I’m very hopeful,” said Mel Martinez, the former senator from Florida, who had supported Mr. Bush. “But it’s a great concern that time has kind of caught up with this whole thing.”