Could Rep. Mike Castle, R-Del., be the next Republican establishment candidate felled by tea party insurgents?

The conservative blogosphere is talking up a challenge to the moderate-to-liberal Castle in a Senate primary on Sept. 14.  Also, the Tea Party Express, which helped direct cash to Alaska Senate primary challenger Joe Miller, is now urging its members to contribute to Delaware challenger Christine O'Donnell.

Castle is the choice of the GOP establishment in the Republican primary. O'Donnell says his support for such things as energy legislation make him too liberal.

The winner will take on likely Democratic nominee Chris Coons for the seat once held by Joe Biden.

Castle's a fixture of Delaware politics who's served as governor and then won 9 statewide elections as the state's only member of the U.S. House.

He is, however, anathema to many conservatives. He gets high grades from the Chamber of Commerce. But he gets poor marks from the American Conservative Union and good marks from the liberal Americans for Democratic Action.