California in spotlight as US voters cast ballots in primaries

Voters have cast ballots in eight US states for key primary elections, with all eyes on California, where the outcome could swing the balance of power in Congress.

Democrats in the country's most populous state are battling to ensure they come out on top in several congressional districts that voted for Hillary Clinton over Donald Trump in the 2016 presidential election.

However, given the state's unusual election system in which the two top vote-getters move on to the general election in November, regardless of party affiliation, Democrats fear they may give Republicans the upper hand in some districts where they have a glut of candidates.

"The worry is that with so many enthusiastic Democrats running for Congress, the Democrats will split up the Democratic vote and you'll end up with one or two Republicans in the top two slots," said Elaine Kamarck, a senior fellow at the Washington-based Brookings Institution.

"So there's quite a bit of concern that this would backfire and that congressional districts that would elect a Democrat in the end would be given only the choice of Republicans."