But when he opened it he had a nasty shock. “You are advised that if your residence in this country is illegal or if you are an immigrant, voting in a federal election is a crime that could result in jail time,” the letter said. It also cautioned that the Government had a computer system capable of tracking voter names — and that anti-immigrant groups across the country had access to the names.

After calling friends he discovered five others who had received the letter — all with Latino surnames and all legal immigrants and naturalised citizens who were entitled to vote. He also found out that no such computer system existed.

The letter, sent to 14,000 Latino registered voters across Orange County in California, and since traced back to a Republican congressional candidate, has kicked off a political storm, exposing ugly undercurrents of xenophobia and racism darkening US politics.

The candidate at the centre of the scandal is Tan Nguyen, an immigrant, who arrived in America aged 8 as Vietnamese boat people fled the Communist takeover. The letter bore the name of a local anti-immigration group but investigators called by Latino officials quickly traced it to his office, prompting angry protestations from his opponents and hasty statements of disassociation from his party colleagues.

The scandal is deeply embarrassing for the Republicans, who have tried hard to disassociate themselves from what critics call an embarrassing history of xenophobia and voter intimidation in the county. During the 1988 race, the local Republican Party was forced to settle a lawsuit after hiring uniformed guards to stand in Latino neighbourhoods with signs reading: “Non-Citizens Can’t Vote.”

Even after the payout, however, leaders still refused to admit wrongdoing. The county is home to several right-wing anti-immigration groups and also the founder of the Minutemen, a vigilante group that sends armed volunteers to patrol the Mexican border.

The row has sparked bad memories further afield, such as allegations, still unresolved, of voter intimidation among blacks in the crucial 2000 presidential battlefield of Florida.

That it appears to have been an immigrant who is responsible for trying to deny other immigrants their vote is, in the words of Mr Nguyen’s opponent, “the most disgusting and sad thing about it”. Loretta Sanchez, the Democratic incumbent, who was born in California to Mexican immigrants, said: “These communities have spent years trying to get naturalised immigrants to vote.”

Republican leaders in the county have called on Mr Nguyen to quit the race.

Arnold Schwarzenegger, the Governor of California, and himself under fire for private comments he allegedly made about Latino immigrants’ failure to assimilate, denounced the letter. Phil Angelides, his opponent, said the campaign was “the latest in a disgraceful pattern of efforts to intimidate Latino voters” by Republicans.

Mr Nguyen still denies knowledge of the campaign and blamed it on one of his staff, whom he dismissed. But his career looks all but over. He has certainly done his party no favours in an area where a third of voters are Latinos.

Democrats are urging the voters to take their revenge at the ballot box. “He owes an apology to these 14,000 people,” Ms Sanchez said.

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