The closer the race between President George W Bush and Senator John F Kerry, the likelier it appears that Nader’s share of the vote — even if he wins only 1-2% of the total — could prove decisive in up to nine closely fought battleground states.

Democrats who blamed Nader for splitting the anti-Bush vote and denying Al Gore victory in the 2000 race are furious that the 70-year-old radical is not only threatening a second upset, but has also accepted financial backing from Republicans thrilled by the prospect of embarrassing Kerry.

At a meeting at Princeton University last week, one of Nader’s fiercest critics handed him a letter urging him to withdraw. David Jones, who has set up a website to monitor Nader’s impact on the race, wrote: “If you choose to continue to divide the president’s opposition, you must know that you are risking the very progressive causes that you have fought for 40 years.”

At the back of the hall where Nader was speaking, pro-Kerry students held up a mock recruiting poster, showing Bush as Uncle Sam above the words: “I want you to vote for Ralph Nader.”

Yet months of Democratic pleas, accompanied by legal challenges to the validity of Nader’s independent status, have failed to sway America’s most stubborn political crusader.

Appearing on a late-night talk show on Thursday, he attacked the “cockamamie” policies of both his mainstream rivals, and insisted that American voters needed “more voices and choices”.

He was even unfazed by a crushing Pennsylvania court ruling that kicked him off the state ballot for allegedly forging the names of almost two-thirds of the supporters who signed his nominating papers. Officials found that the names included Mickey Mouse, Fred Flintstone and John Kerry. Thousands of names appeared with nonexistent addresses.

Judge James Collins described the nominating papers as “the most deceitful and fraudulent exercise ever perpetrated upon this court”. Nader blamed overexuberant supporters but said he was appealing against the ruling.

Nader originally built his reputation as a consumer warrior who forced the car industry to improve safety standards and who led a campaign to have seatbelts made compulsory.

Capitalising on his anti-corporate image, he ran in 2000 as the Green party candidate, winning almost 3m votes — 2.7% of the national total.

Yet Democrats turned bitterly against him after the long ballot wrangle in Florida, where Bush eventually won by 537 votes. Nader won almost 100,000 Florida votes, many of which might otherwise have gone to Gore.

After falling out with the Green party this year, Nader attempted to run either as an independent or as the nominee of the little-known leftist Reform party. Democrats pounced on his organisational difficulties and challenged his right to appear on ballot papers. Forced to fight through the courts, Nader accepted contributions from Republican donors happy to upset the Democrats.

The rows have taken a heavy toll of Nader’s support but he continues to profit from a resilient group of voters who believe that both Republicans and Democrats are beholden to corporate interests.

Terry McAuliffe, the Democratic party chairman, last week repeated his call for Nader to “end the charade” and get out of the race. But opinion polls showed that in several battleground states, Nader may achieve his objective of holding the balance of power. In Iowa, where the two main candidates are in a dead heat, Nader’s share is 4%.

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