What are the mid-terms?
The elections are called mid-terms because they come half-way through the four-year term served by the president, though the polls are in fact for Congress - the two houses of the US legislature. This time, there were also races for 36 of the 50 state governorships, known as gubernatorial elections.
In Congress, all 435 members of the House of Representatives faced the voters, as they do every two years. But only a third of the 100 members of the Senate are up for election at any one time. That translated into 33 seats this year.
Why not elect everybody at the same time?
The American system is designed to be overlapping, both in terms of the powers of the different bodies and in terms of when people are elected to them.
The House of Representatives is the larger of the two Houses of Congress. It was set up as a popular body with the number of members tied to the size of the population. The idea was for it to directly and quickly reflect the public mood, which is why the members face election every two years.
The Senate was planned as a more reflective body, and each state has two senators regardless of its size.
Senators serve for six years. Every two years, one third of them face re-election.
Why are the mid-terms important?
At stake was control of Congress, the legislative branch of the US Government. Republicans had controlled both chambers since 1994, except for a brief time when Democrats held the Senate.
Is incumbency a big factor?
Like incumbent presidents, sitting members of the two houses stand a strong chance of re-election and there are no federal limits on length of tenure.
Robert Carlyle Byrd (Democrat), for example, has been a senator in West Virginia since 1958, while the traditional re-election rate for House of Representatives members is more than 90%.
Apart from the advantage of voter recognition, incumbents across the parties appear to enjoy a clear fundraising advantage.
Sitting members of the House of Representatives net more campaign funds than their challengers by a factor of 7:2 while senators have a 4:1 advantage, according to the non-partisan Center for Responsive Politics.
Business interests account for about three-quarters of all contributions, the centre says.