It may be Indiana or bust for Donald Trump.

If the polls are right, he will dominate in New York on Tuesday and in the coming races across the Eastern Seaboard. He could win nearly all of the delegates at stake — keeping him on a narrow path toward the Republican nomination. That would set him up for what will probably be the most important test of the race: Indiana on May 3.

It may sound strange, but when you start gaming out the rest of the primary contest, it’s hard to avoid the conclusion that his quest to reach a majority of delegates before the convention could all turn on Indiana. If you divvy up the states by expected results, Mr. Trump wins big in the East and West Virginia, loses the winner-take-all rural Western states, and earns his expected share of proportional delegates in Washington, Oregon and New Mexico.

That puts him about 175 delegates short of the required 1,237. Only two real tossup states remain: California (172 delegates) and Indiana (57).