Democrats have been playing offense so far this year as they try to regain the majority in the Senate for the first time in six years. But with five months to go until November’s general election, the Republicans are starting to return fire as they defend 23 of the 35 Senate seats up for grabs this year. This week the National Republican Senatorial Committee (NRSC) – which is the campaign arm of the Senate Republicans – kicked off a major ad blitz in three key races that could determine which party controls the chamber next year. And the NRSC says...