Voters in eight states are heading to the polls in a busy primary election day headlined by a pivotal group California congressional races.

Viable Democratic challengers are targeting seven Republican-held seats in the House of Representatives, with the national party pouring in millions of dollars under the belief that the road to wresting back a majority runs through the Golden State.

But California’s “top-two” electoral system, which allows the leading two to advance to the general election regardless of their party affiliation, has raised the prospect of a Republican shutout.

Numerous Democrats vying for general election spots in a trio of southern California races could split the liberal vote amongst themselves, strategists fear, thus allowing two Republicans to advance and fumbling key pickup opportunities.

Californians will also be choosing finalists to be the state’s next governor. In addition to managing the nation’s most populous state and one of the world’s largest economies, the winner could have a formidable platform to lead the state’s resistance to Donald Trump.

Frontrunner Gavin Newsom, a Democrat and the state’s lieutenant governor, has been a fierce critic of the president - the main question is whether Mr Newsom will face a Republican or a Democrat in November.

Local elections 2018 – in pictures Show all 8 1 /8 Local elections 2018 – in pictures Local elections 2018 – in pictures Jeremy Corbyn outside a polling station in Islington Rex Local elections 2018 – in pictures Theresa May and her husband Philip arrive at a polling station to vote in local elections in central London AFP/Getty Local elections 2018 – in pictures Boris Johnson leaves after voting in London Reuters Local elections 2018 – in pictures Liberal Democrats leader Vince Cable with his wife Rachel at Twickenham polling station Rex Local elections 2018 – in pictures A man enters a polling station as voting begins in local government elections in London Reuters Local elections 2018 – in pictures A man fixes a sign to the door of a polling station as voting begins in local government elections in London Reuters Local elections 2018 – in pictures A poll clerk hangs a polling station sign outside the Victoria and Jubilee Halls in Hartley Wintney, Hampshire, some 45 miles west of London AFP/Getty Local elections 2018 – in pictures Voters at The Vyne polling station in Knaphill, part of the Woking borough, which is one of five councils working with the Cabinet Office to trial the use of ID in polling stations PA

Other contests across the country could help determine which party controls Congress in 2019.

In New Jersey, Democrats are hoping to wrest away the five seats held by Republicans in part by cultivating suburban voters, rehashing a strategy that has helped deliver victories in a series of contests since Mr Trump’s election.

Also drawing attention will be New Jersey’s Republican senate primary. The victor of that race will take on Senator Bob Menendez, a Democrat who has been tarnished by an ethics investigation that led him to be “severely admonished” by a Senate committee for accepting gifts.

Voters in Montana will be choosing which Republican will take on Democratic senator Jon Tester, seen as one of the most vulnerable Senate incumbents.

Mr Trump has repeatedly assailed Mr Tester by name after the senator aired allegations against Mr Trump's pick to be veterans affairs secretary, Ronny Jackson, injecting himself into a contest that could help swing control of the Senate.

Donald Trump: Veterans Affairs secretary-nominee Ronny Jackson should drop out

In Iowa, too, Democrats are targeting incumbent Republican House members and deciding who will take on Republican Governor Kim Reynolds, who recently signed an abortion law widely seen as the nation’s strictest.

Iowa election law dictates that, if no single candidate can win 35 per cent of primary votes, the process moves to a party nominating convention.

Multiple New Mexico Democrats are vying for a chance to challenge Rep Steve Pearce, a Republican who is running for governor.