Republicans' 7-point win in last night's Montana election is great news for Democrats

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.