It appears that many moderate voters, who have long been the biggest obstacle to Mr. Cruz, finally broke his way. According to exit polls, Mr. Cruz won 29 percent of them — far higher than the 12 percent he won in Michigan and 15 percent in Illinois. Mr. Kasich’s share of the vote among both self-described “moderate” and “somewhat conservative” voters dropped.

Perhaps nothing exemplified Mr. Cruz’s newfound competitiveness in moderate areas better than Madison’s Dane County, where he won by a comfortable eight-point margin and 38 percent of the vote. Just a couple of weeks ago, he finished third in places like Ann Arbor, Mich., and Chicago.

Mr. Cruz’s strength did have limits. It was heavily concentrated in the more populous eastern part of the state. In the older, more rural, less religious and less educated western and northern areas of the state, Mr. Trump still ran far ahead of Mr. Cruz.

The distinctly regional element of Mr. Cruz’s strength at least raises the possibility that it won’t be replicated elsewhere. The region is one of the few places in the northern United States where Republicans dominate in well-educated suburbs. (Orange County, Calif., and Indianapolis are similar and still to come.)

But perhaps the best reason to think it still might be part of a broader phenomenon for Mr. Cruz is that he has outperformed expectations at every point since Super Tuesday. He nearly doubled his support in the contests immediately after Super Tuesday, as Mr. Rubio faltered. He posted strong showings on March 15 — like clearing 40 percent of the vote in Missouri and 30 percent in Illinois — that were largely overlooked because it was not enough for him to win.

Those gains put Mr. Cruz very near the point where he could prevent Mr. Trump from earning a majority of delegates — close enough where merely splitting the remainder of Mr. Rubio’s vote would be enough.