ROME — If Italy’s usually politically dexterous nationalist leader Matteo Salvini failed to anticipate one thing, it was that his political rivals would overcome their mutual animosity to join forces and stop him.

But after triggering Tuesday’s collapse of the government with his call for early elections that he almost certainly would win, that’s exactly what seems to be happening.

On Wednesday, the warring enemies of Mr. Salvini, leader of the anti-migrant League party, began the horse-trading to form a coalition government that could relegate the man who has been at the center of Italian politics to the sidelines.

For Mr. Salvini, the danger now is that he could be out in the cold for three years, his momentum stalling and his muscles atrophying until the country is scheduled for its next elections.