EW YORK - For the first time since her Senate campaign began, Hillary Rodham Clinton crossed the Rubicon yesterday, as a statewide poll showed her receiving 50 percent of the vote - the much-vaunted ''magic number'' that thrusts victory into view.
The survey of 889 likely voters, taken by the Quinnipiac College Polling Institute, has Clinton leading her opponent, Representative Rick Lazio, 50-43.
In one sense, the new poll - taken Sept. 20-25 and released yesterday - is as much a psychological landmark as anything else. But psychological warfare is a key component of any campaign and, in that realm, the poll marks a setback for Lazio.
For months now, Lazio and his spokesmen have derided Clinton as an outsider who has no solid appeal to New Yorkers - and, as evidence, have noted that she has never climbed out of the 40s in any poll.
Now she has. As Maurice Carroll, the institute's director, said, ''What a difference a point makes.''
In the poll before this one, taken Sept. 12, Clinton led Lazio, 49-44 - a mere one-point difference from yesterday's tally.
However, this one point is not merely symbolic. Given that both polls have a margin of error of 3 percentage points, the earlier outcome was technically still a tie. Clinton's 49 percent might have really been as low as 46 percent, and Lazio's 44 might have been as high as 47.
With a spread of 50-43, on the other hand, Clinton is ahead even at the outer boundaries of probability - assuming the poll is accurate and measures more than fleeting voter sentiment.
Lazio has been sinking in several polls ever since the Sept. 13 candidates' debate in Buffalo. Several political analysts, surprised at Lazio's aggressiveness and Clinton's relative meekness, gave Lazio the nod in that match. However, most voters, for the same reason, thought the exact opposite.
By 44-34, the Quinnipiac respondents gave Clinton the win. More significantly, nearly half - 48 percent - came away from the debate with a less favorable view of Lazio. Only 18 percent had a less favorable view of Clinton.
The debate seems to have had its deepest impact in upstate New York and among voters who consider themselves independents.
Clinton has had a hard time winning over these groups. But now she leads Lazio among independents, 49-42 - a stunning reversal from before the debate, when Lazio led, 53-42.
More surprising, Clinton trails in upstate New York by only 46-44 - a statistical tie. Before the debate, Lazio had a clear lead there, 50-42.
Upstate - defined as every place outside New York City and its immediate suburbs - is generally the state's most conservative and Republican region.
''If this is the best Lazio can do upstate,'' Carroll said, ''he ought to start looking into setting up a private law practice.''
Of course, the contest is far from over. One-quarter of Clinton's supporters say they might change their mind between now and election day. The same is true for one-sixth of Lazio's supporters.
