Barbara Buono speaks to supporters at a home in Montclair, N.J. on June 26. (Jason Horowitz /The Washington Post)

On the day the Supreme Court struck down a key component of the Defense of Marriage Act, Barbara Buono, a New Jersey state senator and the Democratic Party’s nominee for governor, stood on the deck of a stately home here, overlooking a garden spotted with stone statues of Pan, a turquoise swimming pool and a crowd of supporters wearing “I Support Marriage Equality” stickers.

“There are small windows of opportunity that open when an unexpected event or current happens,” Buono, who is trailing Republican Gov. Chris Christie by some 30-odd percentage points in most polls, told the crowd. “And this is our time. This is our moment.”

Wednesday’s historic ruling mattered not for New Jersey, because Christie had vetoed the Democratic-led legislature’s same-sex marriage bill. The politically adroit Republican governor of a largely Democratic state has argued that same-sex marriage should be decided only by a statewide referendum. But in the hours after the decision, he seemed to escalate his opposition by calling the decision “incredibly insulting” to those who passed DOMA. That sort of rhetoric may earn the ambitious and boisterous governor credibility with skeptical conservatives in early-voting states like Iowa and South Carolina, but it also provided Buono with an opening to take the offensive. That is an exceedingly rare development in a race where Buono has so far served as a human Richter scale. Her devastation will measure the magnitude of Christie’s presidential prospects.

In an interview, Buono acknowledged that despite Christie’s vulnerabilities on property taxes, the economy and jobs, his popularity and her relative anonymity have made the race exceptionally challenging.

“When people throw challenges at me,” she said defiantly, “I double down.”

Get ready for much down doubling.

At least five Democratic officials passed on the offer to take on Christie, 50, before Buono accepted the assignment. A former Democratic governor recently thanked her by suggesting she give up.

President Obama continues to shower Christie in post-Hurricane-Sandy/Mitt-Romney-who? love, meeting personally with the governor on a recent trip but only addressing his party’s nominee in a group. (“I met with the president in a small clutch, which is a new vocabulary word for me,” Buono said.)

The state party denied her a spot at the Democratic National Convention; she sneaked in with a non-New Jerseyan’s floor pass. The party then bucked tradition by not deferring to its gubernatorial nominee’s choice for state chairman, prompting an internecine war when she needed the troops rallying to raise money for her. (“Battles like this go on a lot, but typically under the radar and not played out in the press,” she lamented.)

Christie hasn’t helped. In a premature coup de grace, the governor scheduled a special election to replace the late U.S. senator Frank R. Lautenberg only three weeks before the general election. That means the Democratic voters and fundraisers who are likely to come out for celebrity Newark Mayor Cory Booker (D) will be less likely to come back out and support Buono for governor. (“It was a cynical, arrogant move completely motivated by his unbridled ambition to become president,” she said. “This will depress the vote.”)

And far from relenting, Christie is embracing his doughnut-chomping, lap-belt-surgery celebrity on the national stage with the likes of Bill Clinton and David Letterman while pounding Buono back home with negative ads intended to boost his margin of victory and the case for Christie 2016. He has sought to tether her to the economic travails experienced by the state during the tenure of Christie’s unpopular predecessor, Jon S. Corzine (D), accused by the government of misusing customers’ funds when he ran the investment firm MF Global.

“That’s her record,” said Kevin Roberts, a spokesman for Christie’s reelection campaign, “which is why she isn’t talking about it and why she’s falling further behind in this campaign.”

Even ostensibly good news such as the Supreme Court’s DOMA decision, which shined a spotlight on Christie’s record and, as she put it, his effort “to package himself as something he’s not,” came with a downside. The decision relegated a sympathetic New York Times profile of Buono deep inside a newspaper that’s still influential with many Jersey voters. (“Well, obviously you always want to be on the front page. Who doesn’t?” she said. “What was it? A-20 or something?”)

Despite all this, Buono fancies herself a fighter. A divorced and remarried mother of six (four her own) she grew up in poverty with her two older sisters, an Italian immigrant butcher father and a schoolteacher mother. She overcame brushes with welfare as a young woman to become a lawyer and then the intransigence of party machines to prevail as a successful politician. The young woman who used to sneak across the Hudson into Greenwich Village movie houses and dreamed of being a film critic (“I aspired to be Andrew Sarris”) is now doing everything she can to act the part of a candidate with a prayer.

On Wednesday evening, Democrats and gay rights activists packed the home of a prominent Jersey Democratic donor. Mostly they were excited about seeing former congressman Barney Frank (D-Mass.).

Frank, who grew up in Bayonne, N.J., held court in front of a brick fireplace next to a grand piano. “In theory this was a great day for people in New Jersey, but thanks to Chris Christie, the fact is it won’t mean anything if you live in New Jersey,” he said, pausing to point out his own husband in the back of the room. “Governor Christie in his careful balancing, today I’ll be a conservative, tomorrow I’ll be a moderate, next week I’ll be a model for weight loss, whatever.”

Buono made her own entrance, to less fanfare, and mingled with some of the attendees. She wore a tailored white pantsuit, black-and-white heels and her hair in the Nancy Pelosi fashion. She talked to several people about her daughter’s open letter this month declaring her homosexuality and her disgust with Christie’s moderate reputation despite his socially conservative positions.

“You pulled that card,” one woman said approvingly of the daughter’s statement.

“She gave me permission,” Buono demurred.

Party organizers invited everyone out onto the deck to hear Buono and other elected officials speak. On the way outside, Buono’s communications director briefed her on Christie’s remarks and relayed that the governor had called the decision offensive to the people who voted for DOMA.

“He pulled a tea bagger,” Reed Gusciora, a gay former assemblyman whom Christie once called a “numbnuts,” said in the huddle.

“He really wants to be president,” said Buono, adding, “he’s offensive.”

Buono then stepped to the top of the stair, next to a flowerpot, to deliver her remarks. She is partial to graceful arm gestures and a ballet first position stance on the stump. At times, it seems she is interpreting her own speech with dance. She interrupted her attack on Christie to say, “I was just fact-checking with my communications director,” about Christie’s comments about the Supreme Court ruling, before reporting, inaccurately, “he called it repugnant.” She added, “He said we’ll see what happens on November 5. Well, you know what I think? That sounds like a challenge, don’t you? Are you up for it? I am. I am.”

Buono, who runs five miles a day, then climbed into a white SUV to her last event of the evening, a Democratic dinner with union members in Linden.

In the basement of Amici III, men with gold chains around their thick necks and women with high hair ate baked ziti, drank Bud Lights and red wine, and wiped their mouths with maroon napkins. They all watched as Buono stood in the middle of a dance floor telling the story of her grandma Angelina and her Italian immigrant father who died when she was 19. When she got to 1975 and her college graduation she urged, “Stay with me on this, I have to talk about who I am because that’s why I’m running for governor.”

After her speech, which ultimately went over well, she listened to Felice Twaddle complain about property taxes (“I am paying 8,000 freaking dollars and have two kids in college. I mean really? Really?”). Christopher Lukenda called Christie a “liar” for cutting police pensions, and Cynthia Krushinski urged her “to be heard.” Rich Bobish, a fireman, said she could win if Democrats turned out and then observed, “She’s better looking than Chris Christie, that’s for sure.”

In the back of the room, Buono ran into Ralph Froehlich, the 12-term Union County sheriff who held both her hands.

“I have been an underdog since grammar school, and I’ve never lost a battle,” he said.

“Well, I’ve always been underestimated in politics, and I’ve always defied expectations,” she said.

“That’s a luxury! That’s a luxury! Because that stupid son of a bitch, they talk and they get lazy and . . .”

“Got to use it to your advantage,” she interrupted.

“And then you get ’em. You get ’em,” he said.

“Shhh,” she said. “Keep it a secret. Keep it a secret.”