Not endorsing an incumbent colleague is tantamount to an endorsement of the opponent. At least as close as you can get without breaking long held protocol.
"Long held protocol" in the GOP is what got us Obama and a country that is farther than the founders' intent than ever before. Besides, how about how many times this incumbent has slapped all of his colleagues in the face by buddying up to Kennedy and Feingold and voting against conservatism over and over?