Republicans have the advantages of a friendly turf, the history of this political cycle — which favors the party that doesn’t hold the White House — and the waning popularity of President Barack Obama, who sometimes seems indifferent.
Democrats hope to minimize losses as stronger candidates face a few gaffe-prone opponents, and by benefiting from a superior ground game or voter turnout machine and the waning popularity of the Republican brand.
As the nine-week home stretch starts, Republicans have a tailwind. They are solid favorites to capture three seats of retiring Democrats: Montana, South Dakota and West Virginia. Then, of the eight races both sides agree are very competitive, Democrats are defending six. A half-dozen states — Alaska, Arkansas, Georgia, Kentucky, Louisiana and North Carolina — were carried by Mitt Romney two years ago; Colorado and Iowa went Democratic then. There are only two Republican-held seats subject to serious challenge, Georgia and Kentucky, where the party’s Senate leader, Mitch McConnell, is almost as unpopular as Obama.
Democratic strategists  believe that in a number of states their candidate will outperform polls by a point or two because of a superior voter targeting and turnout apparatus, building on Obama’s campaign infrastructure. Unlike the last midterm elections, in 2010, when Republicans dominated, they predict respectable turnouts from black voters, Hispanics and unmarried women.
Republicans counter that their voters are more enthusiastic and thus more likely to turn out. They acknowledge that the party’s brand name is worse than it was two or four years ago, though they argue that this midterm contest is overshadowed by the president’s negatives.