Congratulations, Republicans! You won the Senate majority! Now, can you hold on to it for more than two years? Looking at the 2016 Senate map, there’s reason for doubt. Republicans will have to defend 24 seats, compared with 10 for Democrats. And the raw numbers don’t even tell the whole story. Seven seats held by Republicans — Florida, Illinois, Iowa, New Hampshire, Ohio, Pennsylvania and Wisconsin — were carried by President Obama in 2008 and 2012. And there is chatter about potential Republican retirements in Arizona and Iowa. If either John McCain or Chuck Grassley decided to call it a...