It would be tough for Mr. Trump to prevail in a one-on-one contest against a typical mainstream Republican, much in the same way that Mr. Buchanan quickly faltered against Mr. Dole.

The catch, though, is that there is no Bob Dole in this race.

There has not been a year when the mainstream candidates have been so weak while factional candidates have been so strong. Perhaps the closest example is the 2004 Democratic primary, but Mr. Dean — a governor who even managed to win the endorsement of Al Gore — was much more acceptable to his party than Mr. Trump and even Mr. Cruz are to theirs. To some extent, this is just a matter of chance: Donald Trumps don’t come around every four years. But it is also a reflection of a Republican Party that is too divided to act in the way that parties usually behave.

The mainstream candidates might not stay this weak forever. Mr. Trump will not be accepted by the party’s establishment, and he will remain unacceptable to a large number of its well-educated voters. This gives the party time to coalesce behind a candidate, like Mr. Rubio. The primary calendar is structured in a way that makes it easier for the party to take its time, too, because relatively moderate blue states come later in the season. The longer it takes for the establishment to elevate a strong candidate, though, the greater the odds that no candidate earns a majority of delegates before the convention.