THE EXORCIST:

When building a statistical model, you ideally want to find yourself surprised by the data some of the time -- just not too often. If you never come up with a result that surprises you, it generally means that you didn't spend a lot of time actually looking at the data; instead, you just imparted your assumptions onto your analysis and engaged in a fancy form of confirmation bias. If you're constantly surprised, on the other hand, more often than not that means your model is buggy or you don't know the field well enough; a lot of the "surprises" are really just mistakes.

So when I build election forecasts for FiveThirtyEight, I'm usually not surprised by the outcomes they spit out -- unless they're so surprising (a Republican winning Washington, D.C.?) that they reflect a coding error I need to fix. But there are exceptions, and one of them came in the U.S. Senate race in Texas between Republican incumbent Ted Cruz and Democratic U.S. Rep. Beto O'Rourke. I was pretty sure that once we introduced non-polling factors into the model -- what we call the "fundamentals" -- they'd shift our forecast toward Cruz, just as they did for Marsha Blackburn, the Republican candidate in Tennessee. That's not what happened, however. Instead, although Cruz is narrowly ahead in the polls right now, the fundamentals slightly helped O'Rourke. Our model thinks that Texas "should" be a competitive race and believes the close polling there is no fluke. [...]

Some factors hurting Cruz have nothing to do with Cruz himself, but rather with the state of Texas. Historically, the incumbency advantage is larger in small, idiosyncratic states and smaller in larger, more diverse ones. This is why Democratic Sen. Mazie Hirono's incumbency advantage in Hawaii is much larger than Cruz's in Texas or Sen. Bill Nelson's in Florida, for example. In addition, Congress's overall approval rating is low, which hurts incumbents in all states and all parties.

Still, Texas is a red state -- redder in statewide elections than in presidential ones, in fact -- and Cruz won by a fairly healthy (although by no means overwhelming) margin in 2012. That ought to be enough to offset a blue national environment as measured by the generic congressional ballot. If you add up the first four indicators in the table -- incumbency, state partisanship, Cruz's previous margin of victory and the generic ballot -- they'd project him to win by about 9 percentage points.

It's the other factors that push the race toward toss-up status, however. When a challenger has previously held an elected office, they tend to perform better with each level higher that office is. To run for Senate, O'Rourke is giving up his seat in the U.S. House, which is a higher office than had been held by Cruz's 2012 opponent, Paul Sadler, a former state representative. Strong incumbents tend to deter strong challengers from entering the race, but Cruz wasn't able to do so this time. Cruz also has a very conservative voting record, one that is perhaps "too conservative" even for Texas. The model actually penalizes O'Rourke slightly for his DUI scandal, but because the scandal has been public knowledge for a long time, the model discounts its importance.

Fundraising is another influential factor hurting Cruz. Ordinarily, you'd expect an incumbent to have a pretty healthy fundraising advantage. Instead, O'Rourke had more than doubled Cruz in dollars raised from individual contributors as of the end of the last filing period on June 30 -- an advantage that will probably only increase once the campaigns file their next fundraising reports, which will cover up through Sept. 30.

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