The 5 big questions about the Senate battleground map

Politico.com - For more and to sign up click here The battle for the Senate won’t be decided for another year and a half. But the key questions that will determine who wins the upper chamber are beginning to come into focus. For Democrats to flip the chamber, everything needs to go right. They have to net four seats, and a wave of retirements earlier this year is expected to make a handful of Democratic-held…