The new poll, released Thursday, has Menendez favored by 49 percent of likely voters, with 45 percent of those polled favoring Kean. Six percent were undecided.
In the prior Quinnipiac poll, released Sept. 20, Kean was favored by 48 percent of likely voters with Menendez at 45 percent.
Both polls have a sampling error margin of plus or minus 4 percentage points, meaning that the difference between the candidates is not clear-cut. Adding to the uncertainty, 18 percent of those polled said they could change their mind before Election Day.
The latest poll of 761 likely voters was conducted Oct. 4-10.
"Even though the lead has shifted in this race, it looks like the two will be neck and neck down the stretch to Election Day," said Clay F. Richards, assistant director of the poll.
In the latest poll, Kean continues to outpoll Menendez on honesty. Kean is viewed as honest and trustworthy by a margin of 51 percent to 18 percent, while voters are split on Menendez's honesty and trustworthiness, 38 percent to 39 percent.
Kean also gets a better favorable-unfavorable rating than his rival. Kean's favorable-unfavorable split is 34 percent to 18 percent, compared with a 32-32 percent split for Menendez.
Kean has tried to make ethics a cornerstone of his campaign, while Menendez has tried to paint Kean as a mouthpiece of President Bush, whose job approval rating remains a low 38 percent.
The war in Iraq is also thought to be the central issue in the campaign. Sixty-three percent of likely voters polled said they disapproved of the way the president is handling the situation in Iraq. Thirty-two percent approved of Mr. Bush's handling of Iraq.
As for progress in Iraq, 55 percent said America is losing the war.
The survey measured the degree to which those campaign tactics may be resonating with likely voters.
More than three-quarters of voters surveyed have heard or read about ethical issues concerning Menendez. Voters are split about whether ethics will influence their vote: 49 percent said they were less likely to vote for the incumbent because of ethics, while 44 percent said ethics would have no effect.
Some 39 percent of the respondents said Kean would support the president's policies "too often" if elected to the Senate, while 9 percent said he wouldn't support the president often enough, and 42 percent said the amount would be "about right."