“US midterm election: How it works, why it's important and races to watch”

Key points

• All 435 House seats are up for election for two-year terms.
• There are 100 Senate seats, with about one-third contested this year.
• Currently Republicans hold slim majorities in both chambers.
• Democrats need four more House seats to claim a majority; four Senate seats for control.
• Notable races include Iowa, Virginia, Montana (House) and Texas (Senate).