Interest group endorsements are critical to a primary campaign. Witness that all major Democrat hopefuls appeared at a construction-workers union conference last week, hoping to curry favor with big labor. The next day, Sen. Hillary Clinton returned to Washington to receive an endorsement from the National Organization for Women (NOW), another pillar of the liberal establishment. NOW's political action committee enthusiastically threw its weight behind Senator Clinton—the first woman with a seemingly real shot at the Presidency—and pledged to rally female voters across the country on her behalf. NOW's "thumbs up" for Hillary is no surprise. Sen. Clinton's domestic policy...