A poll released on Wednesday by Quinnipiac University shows that Sen. Bob Menendez’s (D-NJ) lead over Republican Bob Hugin in the New Jersey U.S. Senate race has dwindled to just six points.

Menendez leads Hugin by 43 percent to 37 percent in the poll conducted between August 15 and August 20, a dramatic decline from the 17 point lead Menendez had in the last Quinnipiac University poll of the race, released in March. The poll has a margin of error of 4.6 points.

“The New Jersey U.S. Senate campaign is developing into a horse race as Democratic incumbent U.S. Sen. Robert Menendez leads former pharmaceutical executive Bob Hugin, his Republican challenger, by a slim 43 – 37 percent,” the Quinnipiac University Poll said in a statement that accompanied the release of the poll:

Menendez was involved in serious wrongdoing, New Jersey voters say 49 – 16 percent, including 38 – 25 percent among Democrats. Among all voters, 31 percent say they haven’t heard enough to decide if Menendez was involved in wrongdoing. Voters give Menendez a negative 40 – 47 percent approval rating and a negative 29 – 47 percent favorability rating. Hugin gets a slightly positive 24 – 20 percent favorability, but 54 percent of voters haven’t heard enough about him to form an opinion. Ethics in government is the most important issue in deciding how they will vote for U.S. Senator, 25 percent of New Jersey voters say, as 21 percent list taxes; 19 percent cite health care, with 14 percent each for immigration and the economy.

“As Sen. Robert Menendez sees his once dominant lead whittled down to single digits, New Jersey voters are sending a clear message. They are troubled by the ethics cloud hanging over him,” Mary Snow, a polling analyst for the Quinnipiac Poll, said of the new poll.

Menendez was indicted on public corruption charges in 2015. In November, the presiding judge declared a mistrial in the case brought against him by the Department of Justice and tried before a jury in a New Jersey federal court. In January, the Department of Justice announced that it would not retry the case.

Though Menendez no longer faces federal corruption charges, his Republican opponent, former pharmaceutical executive Hugin, has successfully prosecuted the case in the court of public opinion, as Menendez’s plunge in the polls indicates.

Menendez is clearly troubled by the Hugin challenge, and has fought back with a series of recent attack ads.

“The commercial began running two weeks before Labor Day, the traditional start of the campaign season. It’s the first of a series of broadcast, cable and online ads to run between now and Election Day,” NJ.com reported:

His initial commercial doubled as both biographical and a way to attack Hugin. He narrated the ad himself, showing pictures of Union City, where both he and Hugin grew up. He criticized his opponent’s record at Celgene, which fought a lower-cost alternative to its Revlimid cancer drug, and for supporting Donald Trump’s presidential campaign. “I never forgot my roots. He has,” Menendez said. “I’m fighting for equal pay and affordable health care; he’s for corporate tax breaks. I’m working to lower prescription costs; Hugin gouged cancer patients. And I’m standing up to Donald Trump. He donated hundreds of thousands to him.”

The Hugin campaign, however, used the Menendez attack ad as a way to once again highlight the long list of ethical problems surrounding Menendez:

In response, Hugin cited Menendez’s relationship with a friend and campaign donor, Dr. Salomon Melgen, who was convicted of Medicare fraud. The Senate Ethics Committee “severely admonished” Menendez for receiving gifts and trips from Melgen while intervening with federal agencies on his behalf. “Bob Menendez’s career of corruption and self-service is an embarrassment to our shared hometown,” Hugin said. “The people of Union City are honest and decent, and Bob Menendez is neither. He sold his office and took $1 million in illegal gifts from a convicted felon, and has failed New Jersey for 25 years in Washington by raising our taxes again and again, letting our infrastructure crumble, and creating a mess in both immigration and health care. It’s time for a change.”

If Hugin’s surge continues, New Jersey may elect a Republican to the U.S. Senate in November for the first time in more than four decades.