WASHINGTON — After toppling three-term Republican Sen. Robert F. Bennett in Utah, tea party activists and other conservative critics shifted their sights Sunday to a mid-May primary in Kentucky, their next big challenge to a political establishment they have vowed to upend. Bennett, 76, left the door open to a write-in campaign after losing his bid for renomination, raising the possibility of an unpredictable three-way race that could yet extend his career. But within minutes of Bennett's defeat, the chairman of the Republican senatorial campaign committee, Sen.