The special election is set for Oct. 16.
A plurality of likely voters also say Booker is seeking the Senate seat to boost his own platform, rather than to serve his constituents, according to the poll.
"While Cory Booker's lead is relatively sizable, many voters have doubts about his motives in seeking the seat," said Patrick Murray, director of the Monmouth University Polling Institute.
The poll comes a week after a Quinnipiac University survey showed Booker with a 12 point lead, though other polls have given him a larger cushion.
Lonegan is scheduled to spend Tuesday campaigning with Texas Gov. Rick Perry at town-hall meetings across New Jersey to rally against Obamacare. The Republican effort to delay implementation of President Obama's health care law helped lead to the federal government shutdown over night.
The Booker campaign used the news of the shutdown to attack Lonegan with a new television ad Tuesday that will run on cable networks in New York and Philadelphia and on broadcast and cable in Atlantic City.
The ad says Lonegan supports the shutdown and paints him as "too extreme for New Jersey," saying he would privatize Social Security and ban all abortions.