Advisers to Mr. Rubio are quick to point out that he has placed second in the last two contests, no matter how slim the margin, and that polls show he remains the overwhelming second choice for most Republican voters. Now, after arguing that the results in Iowa and South Carolina effectively narrowed the primary to a three-man race, his team is suggesting that his finish in Nevada has turned it into a two-man contest with Mr. Trump.

That may be wishful thinking, particularly given his middling finish in Nevada.

But projecting an aura of victory and confidence, even if the election returns suggest otherwise, is an essential element of the Rubio campaign’s strategy. On Saturday night, even before they knew what the results in South Carolina would be — whether Mr. Rubio would finish second or third — campaign officials were acting publicly as if they had won. They distributed news articles about how Mr. Rubio was surging, and they reminded voters of the state’s track record in predicting winners.

Still, the Rubio campaign risks underestimating the breadth of Mr. Trump’s appeal by dismissing him as the latest incarnation of Pat Buchanan, another conservative populist who made a strong showing with Republican primary voters early on in 1992 but ultimately faltered.

Mr. Bush tried to project a similar air of inevitability. Party leaders quickly lined up behind him. The biggest Republican donors signed on. And his supporters were making many of the same arguments Mr. Rubio is now: In a topsy-turvy campaign, he was the safe, sane alternative who would unite the majority of the party that appears to dislike Mr. Trump.

Mr. Rubio himself has been making this argument, optimistically claiming the nomination is his to lose. “I am going to be the nominee,” he said aboard his campaign plane on Monday as he crisscrossed Nevada. “We’ve shown the capability to grow,” he added. “Six months ago, we weren’t at 20 or 25 percent. We were at 6 and 7 and 8 and 9.”

In a telling sign, he now entertains detailed questions about his path to the nomination almost every day, something he had routinely shied from.

Over the past 48 hours, he has rolled out dozens of endorsements from current and former Republican governors, senators and representatives to showcase what his aides describe as a coalescing of responsible and respected party figures around his candidacy. But it can sometimes feel as if Republicans are falling in line, not in love.