WASHINGTON -- It's just one poll, but looks like U.S. Sen. Robert Menendez should prepare for a dog fight.

A poll released Friday shows Menendez with a slim lead over Republican challenger Bob Hugin, the first survey that hasn't given the incumbent Democrat a double-digit lead.

Menendez led Hugin, a former Celgene Corp. executive, 28 percent to 24 percent, among New Jersey registered voters in the Fairleigh Dickinson University Poll. Almost half the electorate, 46 percent, said they were undecided.

"Senator Menendez's recent federal trial and bipartisan admonishment by his Senate colleagues are clearly taking their toll," said poll director Krista Jenkins, an FDU political science professor. "It's not uncommon for incumbents to cruise to reelection, but these numbers suggest he's going to have to woo voters like he hasn't had to in a long time."

In addition, Menendez was in negative territory in the FDU survey with 39 percent disapproval and 32 percent approval ratings. That was better than his performance in October 2017, when 57 disapproved of the senator and just 26 percent approved.

The poll was the first taken after the Senate Ethics Committee "severely admonished" Menendez in April for accepting gifts from a friend and campaign donor, Dr. Salomon Melgen, while intervening with federal agencies on his behalf.

Two other polls taken after federal prosecutors declined to retry Menendez after their first attempt ended in a mistrial and a judge dismissed several charges, gave the incumbent leads of 21 points and 17 points.

The FDU survey was released in advance of the June 5 primary, where Menendez faces community newspaper publisher Lisa McCormick while Hugin is being opposed by businessman Brian Goldberg, who unsuccessfully sought the GOP Senate nomination in 2014.

"It's no surprise the race has tightened," Hugin spokeswoman Megan Piwowar said. "When given the choice between a proven job creator and former Marine like Bob Hugin, and a corrupt ineffective career politician like Bob Menendez, voters will continue to move steadily in our direction."

The ethics committee rebuke came early enough that Menendez "has time before November to reclaim a more favorable place in the heart of his constituents," Jenkins said.

"Both men will pick up considerably more support as the race progresses as the undecideds begin to break for the candidate who shares their partisan leanings," Jenkins said. "With New Jersey a more Democratic state, Menendez is likely to pick up more of this group than Hugin."

U.S. Sen. Chris Van Hollen, who chairs the Senate Democratic Campaign Committee, said he wasn't concerned about Menendez's re-election chances.

"He is a a proven passionate fighter for New Jersey and the people in New Jersey know that," Van Hollen said at a breakfast with reporters sponsored by the Christian Science Monitor. "It's going to be a fight. He knows it. But I think he's confident, and we're confident, he's going to do very well."

Menendez is strongly favored to win by the two Washington-based publications that track Senate races, the Cook Political Report and Inside Elections

Hugin has issues of his own. Under his leadership, Celgene paid $280 million to settle federal charges that it pushed two cancer drugs for uses not approved by the FDA, and that it submitted false claims to Medicare.

And the company has come under fire from President Donald Trump for making it harder for generic companies to get samples of its profitable cancer drug Revlimid so they can produce less-expensive versions.

As for U.S. Sen. Cory Booker, D-N.J., he is viewed favorably by 55 percent and unfavorably by 27 percent.

The poll of 856 New Jersey adults was taken May 16-21 with a margin of error of 3.5 percentage points.

Jonathan D. Salant may be reached at jsalant@njadvancemedia.com. Follow him on Twitter @JDSalant or on Facebook. Find NJ.com Politics on Facebook.