California primary leads eight elections that could help swing control of Congress

Election is pivotal for Republicans looking to end their mounting struggles and gain a stronger foothold in the nation's largest state

Voters cast their ballots in Los Angeles, California (Mario Tama/Getty Images)

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.

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.

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.

Mr Pearce’s decision to run for governor has also opened up his seat, and a handful of Democrats are hoping to win a spot on the ballot to replace him in a district where registered Democrats outnumber Republicans.