An hors d’oeuvre to whet your appetite for tonight’s debate. Good news for the guy who’s led nearly every poll for seven months: He’s now a slight favorite for the nomination over a guy who doesn’t lead anywhere and is having trouble cracking 15 percent.

Donald Trump is the new favorite to win the Republican presidential nomination, Betfair said, less than three weeks before Iowa caucus-goers cast the first votes of the 2016 election.

The odds on a Trump victory are 15/8, or 34 percent, putting the billionaire real-estate mogul as favourite in the market for the first time, Betfair said in an e-mailed statement in London on Wednesday. Previous favourite Marco Rubio widened to 11/5, or 31 percent, while Ted Cruz is 7/2, 22 percent. Betfair didn’t provide earlier odds in its e-mail.

It’s not just Betfair. PredictWise, which combines data from Betfair with data from other sources, also has Trump at 34 percent versus 32 percent for Rubio and 21 percent for the criminally undervalued Cruz, who’s the frontrunner in Iowa as I write this and has certainly run the best campaign of the primary so far. The case for Trump winning is straightforward: He’s not just ahead in 49 of 50 states, he’s surfing a wave of enthusiasm that no other GOP candidate remotely approaches. Yesterday he pointed to the size of his crowds as evidence that, at least at this early stage, his movement is bigger than Reagan’s. And then he said something interesting. “Why would a person stand in line for seven hours [to attend a rally] and then not want to go into a voting booth? It takes 10 minutes. So I think they’re going to vote.” If he’s right, that’s probably endgame for Cruz in Iowa.

But is he right?

But the challenge in Iowa is that historically, caucusgoers — only a sliver of registered voters — have had to be coaxed out by a field team, rather than be counted on to show up and vote on their own. This is especially true of the demographic that supports Mr. Trump: younger voters and others with a low propensity to turn out.

As temperatures plunged to single digits over the weekend, canvassers for Hillary Clinton posted photographs of themselves on social media going door to door in the snow. Meanwhile, Mr. Trump’s volunteers in Davenport, a city where the campaign appears to be better organized than elsewhere, decided it was too cold to go out.

Seven volunteers worked the phones at the Iowa headquarters of Senator Marco Rubio of Florida in a Des Moines suburb one night last week. At the state headquarters of Mr. Cruz, there were 24 volunteers in a room beneath a sign proclaiming a daily goal of making 6,000 calls. The Trump state headquarters in West Des Moines were largely deserted.

How many Trump fans are at the rallies because they desperately want to see Trump become president and how many are there for the show? Nor is it just Trump’s ground game that may be lacking. After promising last month to spend $2 million on ads each week in early 2016, Trump’s ad buys in Iowa and New Hampshire this week amount to less than $700,000. For the life of me, I can’t understand why a guy worth $10 billion would take risks by not emptying his arsenal against his opponents when he has the resources to do so. Trump fans will say that he doesn’t need ads because he gets tons of earned media from interviews and he’s already universally known among voters. But that name recognition and earned media hasn’t been enough to knock Cruz back in Iowa; the race is close there, and Cruz is favored to win it. There’s no good reason why Trump should be holding back on Cruz at this point instead of running ads hitting him on his eligibility, the 2013 shutdown, the difficulties he might face working with a Congress that hates him, and so on. Is this a pride thing for Trump, i.e. he wants to prove that he can win the nomination unconventionally, through sheer charisma, without spending the giant sums that people like Jeb Bush have wasted? Why would you leave anything on the table when you’re this close to winning a major-party nomination?