The 2018 US elections are expected to be the most expensive midterms in history as advertising passes the $1bn (£760m) mark just 100 days before the election.

Campaigns, political parties, and outside groups have purchased television advertising months before the November ballot casting which is hotly contested in several districts and states as Republicans hold a slim majority.

The 2014 midterm elections hold the existing title as the most expensive, when candidates spent a collective $3.8bn in an effort to gain votes.

Candidates running for the US Senate from both parties, according to The Hill, spent approximately $170m on television slots already and reserved future airtime worth $230m.

Democratic candidates for the US House have spent more than $135m and Republicans have already spent or booked time worth $146m.

Those numbers do not include political action committees, lobbying groups like the National Rifle Association (NRA) or National Organisation of Women and also only comprise the national seats.

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It also does not include spending from billionaires like former New York City Mayor Michael Bloomberg, who pledged $80m to help get Democrats elected to Congress and the Koch brothers, the corporate industrialists who have pledged billions to elect Republicans for several years.

In state governor races, the two sides have spent or reserved $340m in television airtime, with the majority – $250m – having already been spent on contentious primary races in states such as Georgia, Michigan, and New York.

The $1bn figure only takes into account what campaigns have spent or booked for advertising. There has been a total of $1.6bn spent on federal seat races on staff, fundraising, and administrative costs according to the Center for Responsive Politics think tank.

That figure does not include gubernatorial races which are not required to file campaign spending disclosures to the Federal Election Commission.