Two years ago, Barack Obama and congressional Democrats rode a wave of voter disenchantment into power. Many Republicans, even in some traditional strongholds, were swept aside. U.S. Sen. Susan Collins, R-Maine, was an exception. She didn’t defeat Democrat Tom Allen. She trounced him, 61-39 percent of the vote, a landslide victory political analysts attributed to Collins’ popularity with moderates, a standing shared by her Maine colleague, U.S. Sen. Olympia Snowe, also a Republican. Two years later, the same moderate image that protected Collins from the blue wave could make Snowe vulnerable to the red one advanced by the tea party....