The Quinnipiac University poll showed 40 percent of respondents in the heavily Democratic state favored Kean while 38 percent backed the Democrat Menendez and 18 percent were undecided.
It is the first time Kean, the son of former New Jersey Gov. Thomas Kean, appeared to have pulled ahead of Menendez in a poll. However, 48 percent of those who selected a candidate said they might change their mind before the November election. The poll of 985 registered voters, taken during July 8-12, has a sampling error margin of plus or minus 3 percentage points.
In a June 15 Quinnipiac poll, Menendez held a 43-36 edge, with 17 percent undecided.
Menendez, a 14-year congressman, was appointed in January by Gov. Jon Corzine to serve out the remaining year of his own Senate term.
Kean, who has trailed his opponent in fundraising, has accused Menendez of being too indebted to Corzine, while Menendez has tried to link Kean to President Bush. Kean has largely kept silent about Bush but describes himself as pro-choice, in favor of embryonic stem cell research and against drilling for oil in the Alaskan wilderness.