Here’s a piece of evidence arguing against the “Democratic crossover votes helped Dave Brat beat Eric Cantor” theory.
With 100 percent of precincts reporting, House majority leader Cantor received 28,898 votes in the GOP primary, to Brat’s 36,110.
In June 2012, against a similarly little-known and underfunded challenger, Cantor won 37,369 votes to Floyd Bayne’s 9,668.
In other words, Cantor lost 8,471 votes from his total in the last primary. It’s not just that new voters out of the woodwork to vote for Brat; it’s that some of his past supporters either didn’t show up or voted for Brat this cycle.
Note that in 2012 there was a contested, if not quite competitive, GOP Senate primary, where George Allen beat Jamie Radtke. In the 7th congressional district that year, 77,169 votes were cast in the Senate primary, and only 47,037 votes cast in the House primary. So quite a few voters — self-identifying as Republicans, at least for that primary day — cast ballots in the Senate primary and didn’t bother to vote in the House primary.
(That 2012 GOP primary was for congressional offices, not the presidential race. Virginia held its presidential primary in March, and only Mitt Romney and Ron Paul qualified for the ballot.)
UPDATE: More useful evidence: “Michael McDonald of the United States Elections Project found similar results analyzing precinct-level data Tuesday night, reporting GOP primary turnout was lowest in the most Democratic-leaning areas of the state.”
Did some Democrats vote in the GOP primary for Brat? Sure, some local Democrats, such as “Cooter” from The Dukes of Hazzard — also known as former congressman Ben Jones — encouraged this, and probably helped Brat achieve his eye-popping 11-percentage-point margin. But Cantor and his allies and supporters are simply fooling themselves if they attribute his defeat to crossover voters.