By any measure, Utah Senator Robert Bennett is one of the most conservative members of Congress. He has championed a flat income tax and the abolition of the inheritance tax. He favors drilling for oil in the Arctic National Wildlife Refuge. The abortion-rights group NARAL gives Bennett a zero rating.
Yet this year, Bennett faces a serious renomination challenge from his right flank. Out-of-state groups are running advertisements denouncing him as a sellout. How did Bennett, of all people, find himself under the gun?
American politics is an intensely competitive...