Midterm elections: US Senate poll in New Jersey crucial for Democrats

Bob Menendez, 64, has held the seat for more than a decade as a powerful force in Washington, while representing a state that has not sent a Republican to the US Senate in 46 years.

But June's Democratic nominating contest exposed his potential weakness: a virtually unknown challenger who spent zero dollars in the race got almost 40 per cent of the vote.

He is now locked in a tough and expensive race with former pharmaceutical executive Bob Hugin, a Republican who has hammered Menendez as weak on ethics after the long-serving senator's federal corruption trial last year.

New Jersey's other Democratic US senator, Cory Booker, seen as a potential 2020 White House contender, spent Sunday crisscrossing the state alongside Menendez, telling voters that re-electing his colleague would help block Republican President Donald Trump's agenda.

The contest is crucial to the Democrats' slim hopes of taking control of the Senate, which depend on winning two Republican-held seats while defending 10 Senate seats in states that Trump won in 2016.

A loss in New Jersey, a Democratic bastion in the US Northeast, could be a fatal blow to those hopes.

Democrats remain favourites to win the 23 seats they need to capture the US House of Representatives.

Menendez's corruption trial ended in November 2017 when the jury could not reach a unanimous verdict.

US prosecutors then dropped the case, but Menendez was censured by a Senate committee for accepting gifts from a wealthy longtime friend in exchange for official favours.

Hugin, a US Marine Corps veteran and the former executive chairman of Celgene Corp, has poured $36 million of his own money into his campaign, airing a barrage of television commercials attacking Menendez.

"If there were any other normal candidate who was not self-funding, I think Menendez would be ahead by 20 points," said Brigid Harrison, a political science professor at Montclair State University. "Money matters."

The Cook Political Report, an independent group that analyzes congressional races, moved the race to "toss-up" from "leans Democrat" 10 days ago.

Menendez in recent years has been a high-profile Democratic voice in foreign policy, weighing in on sensitive subjects from nuclear talks with Iran to US relations with Cuba as the senior Democrat on the Senate Foreign Relations Committee.

The Democratic-aligned Senate Majority PAC has spent more than $7.5 million to boost Menendez, including running ads in the waning days of the campaign, a signal the party is concerned about the incumbent's position.

Another super PAC, Patients for Affordable Drugs Action, has spent nearly $3.5 million to attack Hugin, saying he kept cancer drug prices artificially high.