Donald Trump has 39 percent of the vote in our Pennsylvania polling average, 37 percent in California, and 39 percent in Maryland. If this were February or early March, that would leave him without much to worry about. Even if Trump picked up zero undecided voters, he’d be pretty much guaranteed a win with the rest of the vote divided between a half-dozen opponents.

But those days are over. In Wisconsin on Tuesday, Trump had 35 percent of the vote — the same share that allowed him to win New Hampshire easily in February, and a larger percentage than he got in winning South Carolina. But not only did Trump fail to win Wisconsin — he got crushed by Ted Cruz.

In many respects, this is an old story. One of the main reasons for our initial skepticism of Trump’s candidacy last summer and fall is that his high unfavorable ratings implied he’d have trouble gaining ground as the field winnowed, potentially allowing other candidates to overtake him.

But I think people may under-appreciate the degree to which this is no longer just a theoretical problem for Trump. It’s become an actual problem, as is readily apparent in the data from the states that have voted so far. The threshold Trump needs to win states is increasing considerably faster than the share of the vote he’s getting, which isn’t increasing much at all. Technically, Trump is chasing delegates, not wins, but most of the remaining states award at least some delegates to the statewide winner (and there are still five winner-take-all contests left on the GOP calendar).