Hundreds of thousands of Californians may have mistakenly registered as members of a far-right political party with segregationist roots, which opposes same-sex marriage and abortion and has called for the construction of a Trump-style wall along the US-Mexican border.

An investigation by the Los Angeles Times found that that as many as three in four supposed supporters of California’s American Independent Party (AIP) believed they were registering as independent voters, free of party affiliation. Their mistake could prevent them casting potentially crucial votes in the Golden State’s primary on 7 June.

Among those thought to have ticked the AIP box on their voter registration forms, under the erroneous impression they were declaring themselves independents, are actors Demi Moore and Emma Stone; former boxer Sugar Ray Leonard; Mark Pincus, founder of tech firm Zynga; and Patrick Schwarzenegger, the son of former California governor Arnold Schwarzenegger.

The AIP has almost half a million registered members, more than any of California’s other minor parties combined. It was founded in California in 1967 and nominated former Alabama Governor George Wallace for President the following year. Mr Wallace, who ran on a pro-segregation platform, carried five Southern states at the general election, making him the only third-party candidate to claim any state’s electoral college votes since 1948.

Today, the party only exists in California, where its members make up three per cent of the state’s 17.2 million registered voters. Democrats comprise 43 per cent of the total, Republicans 28 per cent and actual independents 24 per cent. Yet those few lost votes could prove key to this year’s presidential contests, for both major parties.

California awards more delegates to Democratic or Republican candidates than any other state in the US. Democrats allow registered independents to take part in their primary, but members of another party, such as the AIP, will be unable to cast a vote for Ms Clinton or Mr Sanders.

The primary could also be the GOP’s last chance to deny Donald Trump sufficient delegates to secure the majority he needs to sew up the party’s nomination. Bernie Sanders is campaigning hard here to maintain his momentum in what remains a tight race between the Vermont Senator and Democratic front-runner Hillary Clinton.

The Republican primary is closed to all but registered party members, but some of the new AIP members may not have realised this.

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A survey of 500 registered AIP members found more than 70 per cent were unaware of their affiliation. Many were reportedly horrified by their mistake. A spokesperson for Ms Moore told the LA Times she “is not, nor has ever been, a member of the American Independent Party.” A representative for the Schwarzenegger family said the former governor’s 22-year-old son intended to change his registration after being made aware of the error.

The most prominent member of the modern AIP is Markham Robinson, chairman of the party’s executive committee. The party is no longer pro-segregation, but its views remain controversial. Robinson recently wrote, in a typical tweet regarding abortion: “Killing your own unborn child is as unnatural as any homosexual perversion could ever be.”