Ted Cruz was thought to be an easy favorite in Iowa. Now it’s not nearly as clear.

He has trailed Donald Trump in 10 of the last 12 surveys in Iowa, and prediction markets now give him just a 31 percent chance of winning the state. As recently as a couple of weeks ago, Mr. Cruz had an edge in 8 of the prior 11 polls, with a 79 percent chance to win.

There’s still a lot of time before the caucuses on Feb. 1. The debate on Thursday could change things, as could the belated airing of attacks on Mr. Trump from Mr. Cruz’s allies. Mr. Cruz is also thought to have a considerable organizational advantage over Mr. Trump, who may be dependent on low-turnout voters.

But the increasing possibility that Mr. Trump will win the state — in no small part because of an improbable alliance with the party’s establishment — makes Mr. Trump’s path to the nomination far more plausible than ever before.