New Jersey Democratic Sen. Bob Menendez' once-comfortable lead over Republican Bob Hugin has narrowed in recent polls amid withering attacks over his ethics challenges and personal controversies, and now one rating outfit has shifted the Senate race into the "toss-up" column.

Cook Political Report, reflecting how unexpectedly competitive the contest has become, announced Friday it is moving the race from "lean Democratic" to "toss-up."

“The contest isn’t about anything else but Democratic U.S. Sen. Bob Menendez and his ethics problems,” Cook Political Report’s Jennifer Duffy wrote, noting wealthy pharmaceutical exec Hugin has outspent the incumbent by nearly 3-1, with much of that going toward brutal TV ads.

The imbalance, and potential risk for Menendez, has led a major Senate Democratic super PAC to jump in with its own multimillion-dollar ad buys.

Duffy suggested the race has more to do with Menendez’ reputation than Hugin’s appeal: “The biggest threat to Menendez’s re-election is not so much Hugin than it is the voter who goes to the polls and decides to send Menendez a message, much the way many did in the primary when 38 percent voted for his unknown primary opponent.”

The Cook rating remains an outlier, however, amid the ample field of election pulse-takers.

Fox News’ Power Rankings, Inside Elections, and Larry Sabato’s Crystal Ball (at the University of Virginia Center for Politics) list the race as “likely Democratic.”

The Real Clear Politics polling average shows Menendez ahead nearly 8 points, though recent polling reflects a slightly tighter race.

In a deep-blue state, Menendez’ campaign has fought back by trying to tie Hugin to President Trump’s policies, calling him “another vote for Trump.” Boosted by incoming Democratic PAC money, the senator also has vehemently defended himself against the latest Hugin attacks.

The highest-profile of those was a stunning ad that revived allegations the senator had sex with underage prostitutes during past trips to the Dominican Republic

At a press conference earlier this month, the New Jersey senator called the ad “despicable” and “desperate.”

“These are lies,” Menendez said. “I know it. You know it. And Bob Hugin damn sure knows it as well.”

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

“It stoops so low with its bogus lies that it might as well been directed by Donald Trump himself,” the Democratic senator said.

The explosive television ad cited 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 said. “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.)

New Jersey hasn’t elected a Republican to the Senate in 46 years.

Duffy, in announcing the Cook rating change, said “a thumb on the scale for Menendez” remains in the blue state.

But the developments in New Jersey mark another potential setback for Democrats trying to reclaim the Senate majority, coming as North Dakota Democratic Sen. Heidi Heitkamp seems to lose traction in the race against Republican Rep. Kevin Cramer and other races reflect further challenges for the party.

“A status quo election is still possible, but it’s equally likely that Republicans will pick up a seat or two,” Duffy wrote.

Fox News’ Alex Pappas contributed to this report.