Not only will Mr. Cruz struggle to maintain the same level of support among non-Trump voters, but the Northeastern states are also far more favorable to Mr. Trump than Wisconsin. Wisconsin was always poised to be one of his worst states; the same measures suggest that Mr. Trump should approach or exceed 50 percent in some Northeastern states.

The combination of better territory for Mr. Trump and a more divided field could allow Mr. Trump to win in the Northeastern corridor by huge delegate margins, perhaps winning more than 80 percent of them on April 19 (New York) and April 26 (Connecticut, Delaware, Maryland, Pennsylvania, Rhode Island). Mr. Trump could win even more than that if he ends up faring well in deeply Democratic districts with few Republicans and many minorities.

His path is narrow enough that every delegate counts. He might need as much as 90 percent of the delegates from the Northeastern states and West Virginia to keep his delegate target in California manageable. But it’s not very difficult to imagine Mr. Trump doing so well. It’s basically what the polls say he’d get, at least right now.

Indiana

The most important state that no one is talking about is Indiana. The contest there comes one week after the Northeastern primaries, and it’s arguably the most balanced state left in the race. It has a mix of both Mr. Cruz and Mr. Trump’s strengths, somewhat like the two states where they’ve fought two of their closest races so far, North Carolina and Missouri.

Indiana awards its delegates on a winner-take-all basis by congressional district, which could easily allow Mr. Trump or Mr. Cruz to claim most of the state’s 57 delegates with a modest victory.

There isn’t much polling data in Indiana, but Mr. Cruz would be a favorite if he could get the level of support among non-Trump voters that he did in Wisconsin. It would be a closer race than Wisconsin; it’s the type of state where Mr. Trump ought to approach or exceed 40 percent of the vote, not the 35 percent he won in Wisconsin.