US Senate: key battle lines that could change the upper chamber at the midterms

The US Senate is the upper chamber on Capitol Hill. There are 100 Senators, two from each state, and Republicans currently hold a razor thin majority with 51 seats. The US Senate writes and passes laws but has a number of other powers and responsibilities. Among these are approving important presidential appointments, ratifying treaties with other countries and overseeing investigations of officials and public bodies. Senators have six-year terms and just 35 seats are up for re-election. Most of