As far as I can tell, this is profoundly unprecedented. It's not a Majority Leader being defeated in a general election, it's in his own party's primary. And it's not the Senate, it's in the House where having the Majority Leader (on the verge of becoming the Speaker of the House) as your represented confers tremendous power to your district. And they voted him out for one voice among 435.
And it's not like the voters of the district disliked everything about Eric Cantor. By all accounts, it was a one-issue race -- and that one issue was immigration. (Note to voters of Hispanic heritage -- this is one large part of what the Republican Party thinks of you.)
There are questions whether Eric Cantor would stage a write-in campaign. Hints have been dropped, though, boy, it seems unlikely. He was defeated pretty badly, and from what little I know of the district I'd have to think that he'd draw off far more Republican votes than Independent, thereby splitting the vote and giving the election to the Democrat running. But if it's only chance, and he wants it that badly... Well, it's possible, but I'd think not. We'll see.
The biggest question to me is what this will do to other elected Republicans in the House and Senate. I can't imagine it not terrorizing all of them. Whether if pushes them all even far more to the intractable right, or if it wakes them up with such a shock of "What in the name of God have we gotten us into" to the degree that they try to extract themselves from connection with the Tea Party corporations (tm), we'll have to see. If I had to guess, it would be that they think the latter, and do the former.
Above all, it just makes me question the sanity of Republican voters in Eric Cantor's district. To give up all that massive power of a very conservative leader for the sake of a lone voice because of a single conservative issue that's swimming against the tide of history strikes me as one of the more self-flagellating and self-defeating decisions in recent political history.
These are just initial reactions. As more of the story develops, perhaps I'll have more. Or perhaps my head will just explode.