But in general, pundits and analysts tend to overestimate the potential for early-state victories to catapult candidates to the nomination. Momentum helps candidates reach their potential, and in the case of candidates who possess the broad appeal and elite backing necessary to win the nomination, that can be enough. But early wins don’t let candidates overcome their limitations.

It’s something we’ve seen over and over again in recent years. A win in Iowa helped Mike Huckabee win evangelicals for the rest of the race, but it didn’t help him win more secular voters. In 2012, Mitt Romney was a very close second in Iowa and won New Hampshire, and yet still couldn’t break through in South Carolina, where religious conservatives reign, as they do throughout the South. And then of course there was Barack Obama, who won a long string of primaries and caucuses in 2008 and then still ran into a wall of traditionally more moderate and conservative Democratic voters in the South and Appalachia, narrowly holding off Hillary Clinton.

So Mr. Cruz could win Iowa, Mr. Trump could win New Hampshire, and there would still be plenty of room for a candidate who could appeal to the supporters who remain: the party’s mainstream conservative and moderate voters and elites.

The second condition is that Mr. Rubio would need to fill the void and emerge as the natural candidate for those more mainstream conservative voters and donors. The easiest way to do it would be to beat those who are in the more moderate lane — Chris Christie, Jeb Bush and John Kasich — by a clear margin in New Hampshire, either outright forcing them out of the race or encouraging the party’s voters, donors and leaders to coalesce behind his candidacy. He could do this, even while losing the state to Mr. Trump, simply by outperforming the rest of the field by a wide enough margin.

But Mr. Rubio has only a modest lead over Mr. Christie, Mr. Bush and Mr. Kasich in New Hampshire. Any of them could overtake him. All three candidates are paying more attention to the state than Mr. Rubio. And while Mr. Rubio may be a natural choice for mainstream conservatives, he is not such a great fit for moderate and independent voters — the type who supported Mr. McCain in 2000 and 2008, and Ron Paul or Jon Huntsman in 2012. And those kinds of voters are overrepresented in the state.

The third condition is that Mr. Rubio would need to hang on until more favorable states on March 8 and particularly March 15, because he probably won’t be doing much winning before then.

The Republican primary calendar is front-loaded with relatively conservative and Southern states, which makes it very hard for a candidate in Mr. Rubio’s position to finesse an early win.