New York Democrats vote Thursday in a primary for which politicians and pundits have dusted off the old label "too close to call."

But if Mayor Abraham Beame, Rep. Edward Koch, former Rep. Bella Abzug and N.Y. Secretary of State Mario Cuomo all are thought to have a chance to win, the politicians and pundits are certain who will lose - Joel Harnett.

As Harnett pushed himself through the last days of the campaign, he had no illusions. "The polls give me 1 per cent of the vote, but I think I'll do better than that" was the height of his optimism.

American politics has had its share of fringe candidates possessed by an idea whose time very few fellow citizens perceive as having come. Vegetarians, Communists, nudists and Yippies have formed parties. Individuals without parties have heard their own drummers and run for office without any visible support.

But Harnett is none of these. Fringe is not a word that applies to a prosperous businessman whose ideas are in the mainstream of the meager flow of ideas that this campaign has produced. Although this is his first campaign, Harnett has been active in New York affairs for years. He comes from an establishment, but as his campaign has demonstrated, it isn ot the political establishment.

Harnett says he wanted to prove that the political process should be open to all, not just to a small group of people who had become politicians and therefore were an elite fit to present themselves to the voters for each and every office as it fell vacant.

He has proved that the process wasn't very open to Harnett. Early in the campaign the New York Post would list the Democratic candidates and omit his name, he says. The New York Times never sent a reporter to watch him campaign.

Harnett says he made a mistake by not using his limited initial funds to buy advertisiing time on television even though he would have been able to buy very little.

"If you're not on TV, you're not real," Harnett said he learned. "We were out campaigning but no one knew us - we hadn't been on TV - and it was pretty rough."

If harnett gets 1 per cent of the vote, that will be about 7,000 votes at the end of a campaign in which he has spent about $200,000 overall, $50,000 of which was his own money.

Harnett is a weary man at the end of a campaign in which his wife, Lila, was his principal aide and the others wereyoung volunteers, many have already dispersed to their law schools and colleges.

To call Harnett's campaign style lowkey risks exaggerating its aggressiveness. Since June he has traveled the city in a borrowed van, the spray-paint palm trees decorating its side panels partially covered by his posters. He steps from the van, which bears the name "Good Times Machine," to shake hands, give out shopping bags and campaign literature like any other candidate, but there is nothing in his reserved manner, his soft "Hello, I'm Joel Harnett and I'm running for mayor" to stop many of the people hurrying by.

Harnett, 51, is no more a crowd stopper than he is a part of the van generation.

"This is part of the way the city functions," he said in partial explanation of why he entered the race. "I didn't see any way to understand it unless I did it. So I did it." The campaign, he says, has been a logical outgrowth of his long time activities in civic affairs.

He runs the businessman's candidate, but not the candidate of businessmen who are sticking with the better-known politicians. Harnett calls himself a Ralph Nader of city government because of his long criticism of the municipal government and them quickly adds: "I don't mean that I'm in his league."

Harnett does not hype himself, but for all its predictable failure to win him a large following, his campaign has had a major impact.He was the candidate who filed suit to force the Securities and Exchange Commission to release its report on the role Beame, other city officials, banks and underwriters played when they first perceived New York's financial crisis.

The report lambasts Beame. "My most important purpose," harnett replies when asked why he ran, "was to get Beame out . . . to dramatize the ineptnes of the administration. The SEC report did that."

After the SEC reoprt came out harnett was on every local TV news show and people began to recognize him as he campaigned on the streets. They knew he was a real candidate because TV said so. In the last two weeks of the campaign there has also been a series of televised debates between all the candidates and Harnett is good at them. he appears thoughtful, calm and serious, all of which he is. Harnett explains his positions as well as nay candidate.

His stands onwhat have become the major issues of the campaign - he favors the death penalty and would have called out the National Guard during the July 13 blackout - are the same as those Koch has ridden from a poor position in the polls to a virtual tie for the lead.

Koch, however, started a series of television advertisements early and kept them on the air throughout the campaign. Harnett didn't have the money.

As more and more people have decided they will vote for Koch, Harnett has changed from a candidate no one recognized to one who frequently encounters voters who tell him: "Joel, I like you but you're not going to win. Who do you want me to vote for?"

"That hurts," Harnett said. He wants their votes and he has rebuffed approaches from most of the six other Democrats, he says, who want him to drop out and endorse them.

After the votes are counted and the two who finish on top enter an 11-day runoff campaign (it seems certain no vote rquired to avoid a runoff), Harnett says he willw rite a book about New York City and in part about his campaign.

Following the runoff, the Democratic survivor will contest the November election against the winner of the two-man Republican primary between State Sen. Roy Goodman and radio talk show host Barry Farber. Even if he loses, Farber will be on the Party candidate and Cuomo will be on the Liberal Party allot line whether he wins or loses in the Democratic Party primary.