New Jersey’s US Senate election has turned into an honest-to-goodness horse race — and rightly so.

new Quinnipiac Poll shows scandal-scarred Democratic incumbent Bob Menendez leading GOP challenger Bob Hugin by just six points, 43 percent to 37 percent.

That’s down from the 17-point lead Menendez held in the last Q Poll back in March, in a state where Republicans haven’t won a Senate election since Richard Nixon was president.

One explanation is provided in the same poll, which shows “ethics in government” as Garden State voters’ top issue.

Those voters, says Quinnipiac analyst Mary Snow, “are sending a clear message: They are troubled by the ethics cloud hanging over” Menendez.

Democratic voters, recall, sent Menendez the same message in June’s primary, when nearly 40 percent cast their ballots for an unknown who spent no money and didn’t even bother campaigning.

It’s not all that hard to figure out why: Menendez may have dodged a legal bullet when prosecutors opted not to retry him after his first corruption case ended in a hung jury.

But voters seem to have reached their own verdict on the nearly $1 million in gifts and donations he took from Dr. Salomon Melgen, and the favors that Menendez then provided his “pal.”

Favors such as arranging visas for the doc’s “girlfriends” from Brazil, Ukraine and the Dominican Republic, as well as intervening with federal agencies on behalf of Melgen — who was eventually found guilty of Medicare fraud to the tune of $73 million.

You can see why the Senate Ethics Committee admonished Menendez for his conduct, and why Garden State voters by 49 percent to 16 percent told the Q their senator had been involved in serious wrongdoing. Even Democrats agreed, by 38 percent to 25 percent.

Hugin, meanwhile, is running as far as he can from President Trump and most traditional “hard” GOP positions (likely necessary in deep-blue New Jersey), while promising to be “a senator we can be proud of.”

As a senator New Jersey can’t be proud of, Bob Menendez, richly deserves the big trouble he’s in.