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, from ratifying treaties with other countries to overseeing investigations of officials and public bodies.

Senators have six-year terms and just 35 seats are up for re-election. Most of these are currently held by Democrats, making it hard for them to make gains. 

The president’s party could increase their hold of the Senate, since 10 of the Democratic seats are in states carried by Donald Trump in 2016 – five of which he won by double digits. Mr…