Democrats would have to gain a net of four seats if there's a Democrat in the White House — because the vice president can cast tie-breaking votes in the Senate — or five if the GOP wins the presidency. Obama's low approval ratings, a scandal involving Veterans Affairs hospitals, the Ebola outbreak, the rise of Islamic State extremists. [...] presidential elections can favor Democratic congressional candidates by increasing turnout of young and minority voters, and Democrats will not have to spend time distancing themselves from an unpopular incumbent. Democrats are concerned mainly about defending seats in Colorado and Nevada, where Senate Democratic leader Harry Reid faces what could be a bruising re-election fight if he seeks a sixth term. "In the face of what can seem to be a very steep climb for the Republicans you really have to look at each individual race and ask yourself about the vulnerability of each of those candidates," said Ross Baker, a political science professor at Rutgers University. Republicans' fortunes may depend in part on how the newly GOP-controlled Senate functions and whether incoming Majority Leader Mitch McConnell of Kentucky can advance legislation or gets hamstrung by the tea party faction in his caucus led by Sen.