What’s clear is that there isn’t much time for a Cruz comeback. His weak showing on Tuesday might make it even less clear to Republican voters that he is the principal anti-Trump option. The vote in Indiana is in just six days, and early voting is already underway.

If Mr. Trump wins Indiana, a merely modest win in California could be enough to give him 1,237 delegates.

Mr. Trump has led in all of the pre-election polls in California, sometimes by a double-digit margin, with around 40 percent of the vote. A modest victory could allow him to win over 100 of the state’s 172 pledged delegates, and perhaps far more.

In a way, the challenge in California by Mr. Kasich and Mr. Cruz is reminiscent of their challenge nationally. Mr. Trump has been thought to be beatable in California, but neither Mr. Kasich nor Mr. Cruz seems poised to build a coalition broad enough to defeat him.