While I agree with the principle, that is more for the primaries. In the general election, I'll take a liberal with an 'R' over a liberal with a 'D'. The liberal with an 'R' will give us Hastert and company. The liberal with a 'D' will give us Pelosi and gang.
Show me the last time an incumbent was defeated in the primaries and then you'll understand why this "defeat the RINOs in the primaries" is just so much asinine empty rhetoric.