UNION BEACH, N.J. — It was a rare convergence of New Jersey’s governor and its two senators for an official event meant to be about policy, not politics.
But as Gov. Philip D. Murphy announced new programs to help homeowners recover from Hurricane Sandy, there was little doubt why the state’s top Democrats had convened so soon before Election Day.
“I don’t know where we would be without Senator Bob Menendez,” Mr. Murphy said. He was followed by Senator Cory Booker, who offered his own lofty praise: “When I look to the future of this state, we need Bob Menendez.”
Democrats are poised to win a number of governorships across the country and perhaps control of the House on Tuesday, thanks in large part to a suburban backlash to President Trump. But in a state rich with commuters and cul-de-sacs, party leaders are being forced to mount a last-minute, all-hands-on-deck effort to rescue Mr. Menendez’s candidacy and preserve their long-shot dreams of a Senate majority.
In New Jersey, many of the suburbanites who are backing Democratic House candidates from Republican-leaning areas are still uneasy about embracing Mr. Menendez after his 2017 federal corruption trial, which ended in a mistrial. And these voters have been reminded of that case most every day by a monthslong, $30 million ad campaign financed by Bob Hugin, a wealthy former pharmaceutical executive who is Mr. Menendez’s Republican opponent.
So, in an already difficult election year for Senate Democrats, when they are defending 10 states Mr. Trump carried, the party was forced to spend significant time, money and energy attempting to retain a seat in a state Hillary Clinton carried by 14 points.
New Jersey’s leading Democrats — including Mr. Booker, who has been spending much of his time out of state preparing for a widely expected presidential bid — are pleading for voters to rally behind Mr. Menendez. The senator’s campaign has sought to highlight the stakes by broadcasting a commercial portraying a vote for Mr. Hugin as a vote for Mr. Trump.
Allies of Mr. Menendez said his indictment and eventual trial made fund-raising difficult because nearly all of the money he raised was for his legal defense fund. And in the face of a self-funder like Mr. Hugin, Mr. Menendez had no choice but to rely on outside help.
In Washington, Democrats grew increasingly alarmed when in mid-October their own internal polling indicated that Mr. Menendez’s lead had fallen to only two points. Continuing to watch their scandal-tarred incumbent be outspent by seven to one was not a risk national Democrats were willing to take, especially given the prospect that they could take back the Senate in 2020 if they do not lose significant ground this year.
So, perhaps grudgingly, the party’s Senate super PAC began spending in a state where there are 900,000 more registered Democrats than Republicans. And it intended to continue its ad campaign through Election Day.
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The concern among Democrats is being met with unbridled confidence from the Hugin campaign.
“If the election were today, I would have won,” said Mr. Hugin, referring to an internal poll that his campaign would not share publicly, but that he said had him leading by a small margin with 12 days left in the election.
Public polls show that Mr. Menendez is maintaining a single-digit lead, and top strategists in both parties, in New Jersey and in Washington, believe that the Democratic tilt this year in states with high-income demographics like New Jersey will ultimately prove to be the incumbent’s salvation. No Republican has won a Senate election in the state since 1972, and many on the right believe that, no matter the Democrat on the ballot, these kinds of races inevitably prove to be fool’s gold.
“Every two to four years, we see a football, and Lucy, being the New Jersey voters, kind of lifts the football,” said Doug Heye, a former spokesman for the Republican National Committee.
But the fact that state and national Democrats have had to rush to Mr. Menendez’s aid illustrates the shift of strength here to the suburbs — and the dwindling tolerance such voters have for extended exposure to the seamier side of New Jersey’s transactional politics and powerful political machines.
“I’m disappointed that they didn’t put someone better in that situation,” said Joy Wodziak, 63, of Morris Township, during a rally for Mikie Sherrill, the Democratic candidate for Congress in New Jersey’s northern 11th District. “He possibly could have stepped down.”
Ms. Wodziak is the type of voter energizing the suburbs for Democrats like Ms. Sherrill. An “independent for 40 years,” she said she registered as a Democrat this year specifically to vote for Ms. Sherrill in the primaries. She cited the roots of her newfound political motivation in the women’s march the day after Mr. Trump’s inauguration.
But while she is energized by Ms. Sherrill, Ms. Wodziak also vowed to vote Democratic up and down the ticket, for “checks and balances” on Mr. Trump.
“It’s hold your nose and vote,” she said of Mr. Menendez.
State party leaders are worried that even an outsize, albeit reluctant suburban vote may not be enough to ensure Mr. Menendez’s survival, and that he is not wholly on the mind of supporters of Ms. Sherrill and Andy Kim, the Democratic challenger in the razor-thin race in the state’s Third District. While Ms. Sherrill and Mr. Kim are coordinating voter turnout efforts statewide, neither has held a public event with Mr. Menendez since the primary in June.
Mr. Murphy and the state Democratic Party are instead putting an emphasis on the need for an exceptionally strong turnout in northeastern New Jersey, particularly in the Democratic strongholds of Hudson, Bergen, Essex and Passaic Counties. Democrats hold such an advantage in the House districts there that low voter turnout could hamper Mr. Menendez’s efforts.
So on Sunday, with only nine days left before the election, the senator and governor held multiple campaign events in Jersey City, one of the dense population centers that will be key to Mr. Menendez’s re-election.
“Hudson County is incredibly important in terms of turnout to achieve the goal of getting Senator Menendez re-elected,” Mr. Murphy said at a bar in Jersey City, as dozens of supporters snacked on quesadillas, listened to stump speeches and watched the New York Jets lose. “And it’s particularly challenging because there isn’t necessarily a lot of down-ballot competitive stuff in a lot of the communities in Hudson County. So, it puts even more emphasis and impetus on events like this and getting the turnout, particularly among young people. That’s the big plea I have among you all.”
On the campaign trail and on the airwaves, Mr. Menendez has shifted his closing argument to focus nearly entirely on tying Mr. Hugin to Mr. Trump.
“We have a president who has a direct hit on New Jersey,” Mr. Menendez said on Sunday. “We can’t have someone in the Senate who will join him.”
His Republican opponent, knowing how unpopular Mr. Trump is, has been trying to resist having the president wrapped around him.
More and more, Mr. Hugin has been campaigning as an “independent Republican,” a phrase he uses often in front of voters. He even surprised some political observers during the sole debate between the two candidates when he declared, “I am not a Trump Republican.”
While he voted for Mr. Trump in 2016 and donated $200,000 to his election effort, Mr. Hugin has, at times, broken with the president — including a critical statement on Tuesday of Mr. Trump’s desire to end birthright citizenship — as he seeks to establish his own identity.
He has even quoted Edward I. Koch, the popular former Democratic mayor of New York City.
“I’m open about who I am,” Mr. Hugin said in an interview. “I’m not one of these ideologues.”
But to Democratic Party leaders who backed Mr. Menendez so quickly after his trial, tapping into that anti-Trump energy in New Jersey has become crucial.
“We either get this guy re-elected,” Mr. Murphy said in Jersey City, “or, oh my God, the alternative is unthinkable.”