The Quinnipiac University poll shows Booker, a Democrat, with 53 percent of the vote and Lonegan, a Republican who has served as mayor of Bogota, with 41 percent. The election is Oct. 16.
The margin is half as wide as it was in a poll of registered voters last month. But pollster Maurice Carroll cautioned against comparing the surveys because the new poll focused on likely voters rather than registered voters.
Lonegan has been campaigning around New Jersey. His schedule for last weekend included seven public events, and on Monday, he held a "red carpet" event to call attention to Booker's appearances in Hollywood. The same night, Booker was at a California fundraiser where the Hollywood Reporter said actors Ben Affleck and Matt Damon were scheduled to appear.
Lonegan has also blamed Booker for crime in Newark and has called attention to his business dealings, including his role as a founder of a fledgling social media website. Booker has since ended his involvement in that enterprise.
Booker, meanwhile, has kept a lower profile in New Jersey, holding no public campaign events in the state over the past week.
He did respond to Lonegan, though, in a fundraising email sent Tuesday to supporters. "Our opponents are so focused on destructive negativity that they won't pause to think about how they can work with us to solve our toughest challenges," he wrote. "But that doesn't mean we can turn our backs on them - or on anyone who could be part of the solution."
Lonegan, meanwhile, sent out a statement boasting about the poll and his campaign's momentum. "This poll reveals what my supporters and I already knew," he said. "New Jersey is not the hopelessly liberal state everyone thinks it is."
But it wasn't all good news for Lonegan: Forty-three percent of voters said they did not know enough about him to form an opinion.
Quinnipiac's Carroll said that while the race may now be tighter than expected, Booker still has a "comfortable" lead, which is likely to be enough to win in a state that has not elected a Republican to the U.S. Senate since 1972.
The telephone survey of 948 voters conducted Sept. 19-22 had a margin of error of plus or minus 3.2 percentage points.
Booker received the majority of his backing from women, who support him 60 percent to 34 percent. Men and independents are split more evenly.
The winner of special election will complete the term of Frank Lautenberg, the Democratic senator who died in June. The seat will be on the ballot again next year.