EW YORK - In her juggernaut toward the final weeks of her Senate campaign, Hillary Rodham Clinton has neglected one thing: She's not the Democratic candidate just yet.
There's a primary tomorrow, and she faces an opponent. Turnout is certain to be extremely low - very few people know a primary is happening - but that is precisely the danger.
The nightmare scenario going through some party leaders' minds is this: What if scads of anti-Hillary Democrats show up to cast protest votes, while nearly everyone else stays home? Could the first lady get tossed out in a fluke?
Her challenger is a 39-year-old Manhattan orthopedist and Harvard Medical School graduate named Mark McMahon.
Almost nobody thinks McMahon, an unknown, will win. But if he gets a large share of the vote - some put the threshold at one-third - Clinton will be at least embarrassed, perhaps crippled.
Four congressional districts are holding seriously contested Democratic primaries for House seats. Two are in largely black neighborhoods in New York City, where Clinton is very popular. Nearly everyone who turns out to vote there is expected to pull the lever for her.
However, one is in a part of Brooklyn that includes an Orthodox Jewish neighborhood, where she is widely distrusted. The other is in Suffolk County, Long Island, home turf of her Republican opponent, Representative Rick Lazio.
Many voters who turn out for the House contests in those districts will happily stick a needle in Clinton while they are at it.
An insurgent candidate who had money and political acumen would exploit this situation, targeting resources where the leader is vulnerable. Lucky for Clinton, McMahon seems not to have much of either.
In a phone interview from Buffalo on Friday, McMahon acknowledged: ''We're really not targeting anyplace or anybody. We're just going wherever.''
Today, he is scheduled to walk around with Hasidic supporters in a section of Brooklyn - but not where the heated House primary is taking place. He will also go to Long Island - but, again, not in the district where the House contest will boost turnout.
McMahon's campaign has only $240,000, three-quarters of it his own money. ''We've had no formal fund-raising effort,'' he noted. He produced some radio ads, but had not decided - four days before D-day - whether to air them.
He has no background in public life, no standing in the party apparatus. On the issues, his distinctions seem minor. ''I feel more akin to Bill Clinton than to Hillary Clinton,'' he said.
In the unlikely event that McMahon wins tomorrow, Clinton will not be out of the race. She is also the nominee of the Liberal Party, which has a prominent line on New York ballots. Nor is it likely that the state Democratic organization, which has so much riding on her, would abandon her.
Tomorrow's contest was not supposed to happen. Early last year, when party leaders begged the Clinton to enter the race, they promised her a clear path to the general election. There would be no primary, they told her, and persuaded Nita M. Lowey, a Long Island congresswoman who was planning to run for Senate, to back out.
This tactic, ironically, is what persuaded McMahon to jump in. ''That struck me as being very undemocratic,'' he said.
At July's state Democratic convention, when party chairwoman Judith Hope rhetorically asked whether there were any other candidates besides Clinton, a delegate named James McManus shouted out McMahon's name. Hope ignored him.
McManus, whose dentist is McMahon's father, is head of a Democratic Club on Manhattan's West Side. These clubs once held more power than they do now. But Hope's blithe dismissal irritated him.
He and McMahon threw some money into a petition drive and gathered 44,107 signatures, nearly three times the number needed to get on the ballot.
As proof that this development worried the party leaders, the Democrats on the State Board of Elections let four other candidates on the ballot, as well, even though their petitions fell short. The tactic was clear: If there were several unknown candidates, the anti-Hillary vote would be diluted among them. McMahon challenged this ruling in court and won - the others were removed from the ballot. Tomorrow, McMahon will be the ballot's only un-Hillary.
