Republican Bob Hugin is going nuclear against incumbent Democratic Sen. Bob Menendez in the final days of the closer-than-expected New Jersey Senate race, reviving salacious allegations that the senator had sex with underage prostitutes during past trips to the Dominican Republic.

Menendez has long denied the allegations as a smear since they first surfaced on a news site in 2012.

But Hugin, a wealthy pharmaceutical executive, is now running an explosive new television ad that cites papers filed in federal court by prosecutors in 2015 acknowledging the FBI probed the prostitution allegations as part of its corruption case against Menendez.

“It’s right here in this shocking FBI affidavit,” the ad’s narrator says. “President Obama’s Justice Department had evidence that for several years, Senator Menendez had been traveling to the Dominican Republic to engage in sexual activity with prostitutes – some of whom were minors.”

Prosecutors declined to bring charges related to the prostitution claims, though they did pursue a bribery case against him. A mistrial was declared last year after the jury failed to reach a verdict in that case.

In 2015 court papers, prosecutors said the FBI investigated Menendez and donor Salomon Melgen after being presented “with specific, corroborated allegations” they had sex with underage prostitutes, though they decided against bringing any charges on those claims. (Melgen was later convicted in a separate Medicare fraud case, and sentenced to 17 years in prison.)

“Presented with specific, corroborated allegations that defendants Menendez and Melgen had sex with underage prostitutes in the Dominican Republic, the Government responsibly and dutifully investigated those serious allegations,” the papers stated, in response to claims from the defendants that the allegations were "false" and tainted the case.

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The prosecutors also wrote: “Confronted with corroborating evidence of such serious crimes, it would have been an inexcusable abdication of responsibility not to investigate these allegations.”

A Menendez campaign spokesman did not immediately return a request for comment from Fox News. But campaign spokesman Steve Sandberg called the ad a “disgraceful act” in a statement to the Star-Ledger in New Jersey.

"It is clear that Bob Hugin will say, do and spend whatever he wants to try to deceive New Jersey voters and buy a Senate seat," Sandberg said. "This is a disgraceful act by a desperate man. They are lies and he knows it."

The Hugin ad’s narrator also takes a swing at Menendez over claims made at the time by Menendez’s lawyers, as documented in the 2015 court filing, who argued “such easily disprovable” allegations of cavorting with underage prostitutes “would hardly be a federal crime even had it been true.”

“As an initial matter, it is most certainly a federal crime to leave the country for the purpose of engaging in a commercial sex act with a minor, and the defendants’ suggestion to the contrary is unsettling,” prosecutors wrote in response.

New Jersey hasn’t elected a Republican to the Senate in 46 years. But Hugin has been spending millions on negative ads reminding voters of Menendez’s past ethics troubles.

For the New Jersey senator, the race represents the latest in a series of dramatic fights for his political life.

Had he been convicted last November, Menendez could have faced expulsion from the Senate. Having survived the trial, Menendez famously suggested he was out for vengeance.

"To those who were digging my political grave so they could jump into my seat, I know who you are and I won’t forget you,” the Democratic senator said last year.