Prior to the crucial midterm elections of 2018, a handful of states around the US are holding special elections between April and December of 2017. Democrats are eager to mobilise voters in a quest to gain control of Congress after losing to Republicans in November.

IBTimes UK has rounded up everything you need to know about the special elections that will determine the composition of the House of Representative for the next year and fill the Senate seat left behind by Attorney General Jeff Sessions.

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How many elections are there?

There are six special elections being held for five seats in the House and one seat in the Senate. Elections are being held in Alabama, California, Georgia, Kansas, Montana, Pennsylvania and South Carolina.

When are the elections and who is involved?

The special election in Kansas to replace Mike Pomeo (R), who is now the director of the CIA, in the 4th District was held on 11 April 2017. The closely fought race pitted Republican Ron Estes and Democrat Jim Thompson. Estes defeated Thompson 52.5% to 45.7%.

South Carolina is set to have a runoff election to determine the Republican candidate vying for the state's 5th District on 16 May. State Representative Tommy Pope and former state legislator Ralph Norman were forced to return for a runoff on Tuesday (2 May) when neither candidate won majority of the vote.

The winner of the runoff will face off against Democrat Archie Parnell, who has a head start in his long-shot bid to turn the district blue, Politico reported. The general election for the seat previously held by Mick Mulvaney (R) will be held on 20 June.

Montana's special election for the at-large House seat previously held by Interior Secretary Ryan Zinke will be held on 25 May. Both parties have made significant investments in the closely monitored special election, as Democrats look to flip the seat in their favour.

The Hill reported that a GOP super PAC has recently launched attack ads against Democratic nominee Rob Quist over his personal financial issues. Quist, who received the ringing endorsement of former Democratic presidential candidate Bernie Sanders, will face off against Republican Greg Gianforte.

California will hold its special election to replace Xavier Becerra (D) on 6 June. Robert Lee Ahn and Jimmy Gomez, the two candidates racing to fill the seat left behind by California's new attorney general, are both Democrats.

Georgia Special election
Democratic candidate Jon Ossoff greets supporters after the League of Women Voters' candidate forum for Georgia's 6th Congressional District special election to replace Tom Price, who is now the secretary of Health and Human Services, in Marietta, Georgia, on 3 April 2017 REUTERS/Bita Honarvar

Perhaps the most contested race of the year will be held in Georgia, on 20 June. Democrat Jon Ossoff and Republican Karen Handel will face off in a runoff after failing to win the majority of the vote on 18 April. The candidates are hoping to replace Tom Price (R), now the secretary of health and human services, in Georgia's 6th District.

Ossoff narrowly missed taking the seat in April, winning a surprising 48.1% of the vote, while Handel came in second with 19.8%. The Democratic newcomer has earned the support of Democrats nationwide and has reportedly also attracted the support of a small group of Republicans. New analysis by Optimus Consulting shows Ossoff captured 8 to 10% of the GOP's votes in the primary, Politico reported. The 20 June election will be the one to watch.

The special election to replace Attorney General Jeff Session's Senate seat for Alabama will be the last election of the year. Primary elections will be held on 15 August and the general election will be held on 12 December.

Why are the elections so important?

According to The New York Times, Democrats must win 24 seats in the House by the 2018 midterm elections to regain control of the lower chamber. If they manage to turn the House, Democrats will have the power to curtail the Trump administration's agenda. The special elections will also provide a window into what may happen in 2018.