Greg Gianforte's 7 percentage point win in the Montana special election keeps a seat in Republican hands, but fundamentally represents bad news for the GOP. The basic issue, as David Wasserman breaks down for the Cook Political Report, is that for prognostication purposes you don't just want to know who wins or loses a special election — you want to know the margin.
Montana is considerably redder than the average congressional district. According to Wasserman's calculations, in an election where Democrats got 50 percent of the two-party vote nationwide, you'd expect them to get just 39 percent in Montana. Quist scored 44 percent, and with the Libertarian pulling in 6 percent, his share of the two-party vote is more like 46.
Things aren't as simple as saying that Rob Quist outperformed the 39 percent benchmark and therefore Democrats are on track to win — geography means Republicans can hold their majority with less than 50 percent of the vote — but the GOP underperformed badly in Montana, after a similar underperformance in the special election for Kansas's Forth Congressional District.
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There are 120 Republican-held House seats that are more GOP-friendly than Montana's at-large district. If Republicans are winning in places like Montana by just 7 percentage points, then they are in extreme peril of losing their House majority in November 2018.
Republican leaders have taken their party on a risky course, and they ought to strongly consider turning the ship around.