Dave Brat beat Eric Cantor in the Virginia Republican Primary -- a remarkable event. It's very difficult to unseat an incumbent in any case. The Republican establishment is so established that it is indeed remarkable that a challenger could do it. There's no shortage of articles attempting to explain why. But the Daily Caller's Christopher Bedford has a different take. See The Media Is Missing The Point Of Cantor’s Defeat:
“Amnesty was only part of Dave Brat’s victory,” longtime conservative leader and “Takeover” author Richard Viguerie told TheDC. ...
It was the attempts by the Cantor team and other establishment Republicans in Virginia to bully, exclude and defeat grassroots conservatives in his district and elsewhere that made his defeat essential to send a message to Washington,” conservative Virginia activist and writer Mark FitzGibbons told TheDC.
Seems that Republican Party participants use a tactic called "slating" to try to get control of the nominating mechanism in Virginia. Slating -- it sounds like a throw back to Reconstruction days -- is when a large number of people agree in advance on a list of names of people they want to represent them in a convention. When that group votes and they are the majority, their slate wins.
Cantor's group did it and won the first time. But he ticked off a lot of people, so when conservatives and Tea Party activists went to work they had an easy time getting out the vote.
So amnesty was only part of the problem. Cantor dissed his constituents, tried to stack the deck, and ultimately lost the vote.