They are using every possible legal means to deny voters the chance of voting for Mr Nader, including advising Democrats not to sign his petitions to get on the ballot, challenging the signatures he does get, and showing ads attacking his candidacy.
Mr Nader is running as an anti-war and anti-corporate candidate.
A recent email from a Texas Democratic party official to members, obtained by the Guardian, was headed Keep Nader off the Texas Ballot and continued: "We need to make sure he is nowhere near a ballot in Texas."
Other pressure groups attacking Mr Nader have grown out of the primary campaigns supporting Wesley Clark and Howard Dean.
Michael Frisby, communications director of the Stop Nader Campaign, said: "The point is not just to keep him from getting on the ballot but to make him spend money and time in all of these places so he has less money and time to spend getting votes."
The rules for what is required to get on the presidential ballot vary from state to state. In most of them established parties which obtain a certain proportion of the popular vote are included automatically but independents must submit a petition with a certain number of signatures. Five months before the election Mr Nader's name is not yet on the ballot in any states.
A representative of his campaign said the opposition's strategy was misguided, futile and undemocratic. "They're playing a game of expecting us to drop out and that's just not going to happen," Kevin Zees said. "They should be working at getting out there and being competitive."
Mr Frisby said his aim was to convince Nader supporters that the issues they held most dear were those most under threat from a Bush victory.
With the country evenly split between Mr Bush and his Democratic challenger, John Kerry, Mr Nader's candidacy could prove crucial. Opinion polls in at least half a dozen swing states show that Mr Kerry would beat Mr Bush in a two-horse race but lose if Mr Nader was on the ballot.
"If this race is as close as I expect it to be, Nader could get a half, or a third or a fifth of the vote he got last time and be decisive again," Charles Cook, a political analyst in Washington, told the Los Angeles Times.
Before meeting Mr Nader recently Mr Kerry said he would never ask another candidate to abandon an election bid, but hoped to "reduce any rationale" for Mr Nader's candidacy.
"In the end I hope I can make people aware that a vote for Ralph Nader is a vote for George Bush," he said. "A vote for John Kerry is a vote for the principles and values they care about."
After the meeting Mr Nader said: "He's a gentleman and he understands we all have to do what we have to do, as he put it."
There is no evidence that Mr Kerry's campaign is directly involved in these efforts, but senior Democrats in Congress and in his campaign privately concede that they are in favour of keeping him off the ballot.
"We want all the votes for John Kerry we can get," Mr Levigne said.
"A vote for Ralph Nader is like a vote for George Bush so it really doesn't make any sense for us to have him on the ballot."
Mr Nader's supporters and the Democratic party have clashed repeatedly since the 2000 election, which many Democrats accuse Mr Nader of handing to George Bush.
In New Hampshire and Florida, where Mr Bush won by narrow margins, Mr Nader won a substantial number of votes.
Mr Nader's supporters say the polls showed that two thirds of those who voted for him would otherwise not have voted and 10 times as many registered Democrats voted for Mr Bush as voted for Mr Nader.
But Mr Bush's record, particularly with the war on Iraq, and Mr Nader's failure to make an impact on the anti-war movement have persuaded a number of former Nader voters, including the film-maker Michael Moore, to back Mr Kerry this year.
In Oregon, where the Democratic candidate Al Gore narrowly won in 2000 and Mr Nader received 4 per cent of the votes, the Stop Nader Campaign recently ran ads asking: "Ralph, what's more important, your nation or your ego? Don't do this again."
Another anti-Nader group, the National Progress Fund, has been running ads in Wisconsin and New Mexico, where Mr Gore won narrowly and Mr Nader took 4 per cent and 5 per cent respectively.
"Four years ago I supported Ralph Nader because he stood for the issues I believe in," said Bob Schick, a Maryland school teacher.
"I feel I made a mistake. By supporting Ralph Nader I actually helped George Bush."
Mr Nader believes that the Democratic party is trying "to block an effort that reminds them of their past as a party".
"Why don't the Democrats go after the 8 million Democrats who voted for George Bush in 2000?" he said recently.
"Thirty-five per cent of union members voted for George Bush in 2000."
So far by a mixture of poor organisation and a late start Mr Nader has done a fairly good job of keeping himself off the ballot.
In Oregon, where a nominating convention of 1,000 voters would have sufficed to put him in contention, only 741 people showed up.
In Texas he missed the deadline and is now suing the state, claiming that its requirements are unconstitutional. The Stop Nader Campaign will be sending lawyers to oppose him.
His other best chance is to be endorsed by parties which are already on the ballot in certain states where they are particularly strong.
He was recently endorsed by the Reform party, set up by the millionaire Ross Perot, which could put him on the ballot in seven states, including the battlegrounds of Florida and Michigan.
Delegates to the Green party conference this month will decide whether to endorse him again - it was with their support that he ran in 2000. If they do he could be on the ballot in the swing state of Wisconsin as well as California.