It took Republicans three tries to win control of the US Senate and consolidate Congress under their leadership. In the end, the landscape left by the 2008 Democratic triumph left them uniquely vulnerable in the sixth-year midterm of an unpopular presidency. The tables will turn in 2016, warns National Journal’s Charlie Cook, as the gains of 2010 will make Republicans highly vulnerable to losing control of the Senate once again: Today, The Cook Political Report puts two Republican-held seats in the “Toss Up†category—the open seat in Florida and Kirk in Illinois. That toss-up column will certainly grow as...