Nate Silver: “It’s true that other measures aren’t as bad for Republicans as these special elections — for instance, they trail Democrats by ‘only’ 8 or 9 percentage points on the generic congressional ballot, which suggests a close race for control of the House this year that only narrowly favors Democrats. By contrast, the 16- or 17-point5 average Democratic overperformance in special elections so far suggests a Democratic mega-tsunami.”
“But those special election results consist of actual people voting, whereas generic ballot polls are mostly conducted among registered voters — or sometimes all adults. (Very few pollsters will apply their likely voter models until later this year.) In midterm years, polls of likely voters sometimes show a substantial gap from those of registered voters — there was about a 6-point enthusiasm gap favoring Republicans in 2010, for instance, which took that year from being mildly problematic for Democrats into a massive Republican wave that saw them pick up 63 House seats.”
“That sort of turnout gap suggests that registered-voter polls could be underrating Democrats in this year’s midterms — and could turn a challenging year for Republicans into a catastrophic one.”
