Following the midterm returns involves a good deal of math. Here’s a pocket guide to the balance of power at the state and congressional levels, and what seats are on the line, to help you on Election Day.

Battle for Senate

The current balance of power is 55 Democrats (including two independents who caucus with them) and 45 Republicans.

Thirty-six seats are on the ballot on Tuesday – 21 held by Democrats, and 15 held by Republicans. Republicans need six seats to win control of the Senate.

Battle for House

The current balance of power is 234 Republicans and 201 Democrats (including vacancies).

Democrats would need 17 seats to gain the majority, and political analysts generally agree that is very unlikely to happen.

The president’s party has only gained House seats in the midterm elections three times since the Civil War, most recently in 2002. This year, Republicans are poised to gain at least five seats, but could easily gain more.

Gubernatorial Races

Nationwide, Republicans hold 29 governorships and Democrats hold 21.

This year, 36 seats are up for grabs – 22 held by Republicans, 14 held by Democrats. Eight of them are open seats.

This is a tumultuous year for gubernatorial races, and polls show more than a third of the races are competitive, meaning nearly a dozen governors could lose. It’s unlikely that many would go down, but there’s a chance – a small one – that it could be a historic year in that regard.

The biggest gubernatorial wipeout in modern times came in 1962, when 11 of 26 incumbent governors lost.