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The Glass-Half-Empty Case Against Ted Cruz

DES MOINES — Like so much else at this point in the campaign, Wednesday morning’s Des Moines Register/Bloomberg Politics Iowa poll could be looked at in either of two ways for the leading candidates. For Ted Cruz, the glass-half-full interpretation is that the best poll in the state still shows him ahead — by 3 points over Donald Trump — when several other recent polls in Iowa had shown a narrow advantage for Trump instead. The glass-half-empty interpretation is that Cruz’s lead is diminished: The previous DMR/Bloomberg poll, taken a month ago, had Cruz ahead by 10 points.

Our Iowa Republican forecast model splits the difference between these two hypotheses, with Cruz’s position essentially unchanged from where it was before the new poll.

And yet, in the conventional wisdom about who’s going to win Iowa, there’s a lot of glass-half-fullism for Cruz. The prediction market Betfair puts his chances of a win at 65 percent. By contrast, the polls-plus version of our forecast, which accounts for endorsements and the candidate’s standing in national polls in addition to state polls, puts Cruz’s chances at 50 percent (as compared to 26 percent for Trump and 17 percent for Marco Rubio). The polls-only version of our forecast puts Cruz’s chances at 42 percent, essentially tied with Trump.

Our forecast models are blunt instruments in the primaries. They’re not directly accounting for Cruz’s apparently superior Iowa ground game or the fact that he has a lot of support among evangelical voters. Cruz also has among the top favorability ratings in the state and is a lot of voters’ second-choice candidate, putting him in a good position to pick up support should supporters of other candidates waver. That’s the glass-half-full case, and it’s a perfectly reasonable one.

What about the glass-half-empty case?

  • Cruz may not be leading at all depending on what poll you look at; several of them have Trump ahead instead.
  • It’s still awfully early in Iowa. There are almost three weeks of campaigning left to go — and unlike in 2012 or 2008, the stretch run won’t be interrupted by the holidays. That’s an eternity in a race where about half of voters say they’ve yet to make up their minds.
  • Iowa isn’t necessarily a two-horse race. Rubio, sitting in the low- to mid-teens in the polls, is in no worse a position than John Kerry was before winning the 2004 Democratic caucuses, or than Rick Santorum was before winning the Republican race here in 2012.
  • Cruz’s campaign has tended to raise the media’s expectations for how he’ll perform in Iowa. That can be a high-risk, high-reward strategy, reinforcing momentum when things are going well, but potentially triggering “Did Cruz peak too soon?” storylines if he has a rough debate or a bad few days in the polls.

I’m not sure which case I find more persuasive. I agree with the notion that Cruz is the favorite in Iowa, but just how much of a favorite is harder to say.

Check our our live coverage of the Republican debate.

Nate Silver is the founder and editor in chief of FiveThirtyEight.