WESTBURY, N.Y. — Jon Cooper (D) feels a little bit like the last man standing.

With better-known potential Democratic primary challengers to appointed Sen. Kirsten Gillibrand (D-N.Y.) falling by the wayside in rapid succession, Cooper — a member of the Suffolk County legislature, political donor, wealthy businessman and gay rights activist — is ramping up his exploratory effort.

In an interview late last week at the Long Island factory that he owns, Cooper said he is on the verge of hiring a fundraiser, a pollster and a media adviser, and is beginning to travel around the state in earnest. He will make a final decision on whether to run sometime after November, when he is expected to win a sixth and final two-year term to the county legislature.

Cooper is, at the same time, beginning to lay out a rationale for his potential candidacy and talk at length about his own personal story — which is far from the typical politician’s.

“Right now, I’d say if I had to make a decision today, I’d do it,— he said.

While the prospect of a challenge from a little-known lawmaker who represents a small district at the eastern end of the state may not yet be making Gillibrand nervous, she has been moving at warp speed during the August recess to meet as many constituencies and consolidate as much support as she possibly can. She also continues to be a fundraising machine, banking $3.3 million as of June 30 — a figure that could double by year’s end.

Equally significant, the very people whose pictures are displayed prominently on top of a cabinet in Cooper’s office — like President Barack Obama and Sen. Charles Schumer (D-N.Y.) — want Gillibrand to run unopposed in the primary. But Cooper says he cannot be pressured out of the race.

“I’m not in Congress,— he said. “They can’t offer anything and they can’t threaten anything.—

The genesis of Cooper’s primary bid began in January, when he and Rob Cooper, his partner of 29 years, watched New York Gov. David Paterson (D) on television announce that he was appointing Gillibrand, then an upstate Congresswoman, to the Senate.

“Rob said, Isn’t that [former Republican Sen.] Al D’Amato standing next to Gillibrand?’— Cooper recalled. “I couldn’t believe it at first. I said, Nah, it must be someone who looks like Al D’Amato.’ But then I realized it was him. That was a red flag, not just for me but for other progressives.—

Then Cooper said he began to learn about Gillibrand’s record and found it was in many ways diametrically opposed to his. Cooper is something of a hero to Long Island liberals. He has sponsored a local hands-free law for cell phone use while driving; sponsored anti-drug, anti-tobacco and anti-loitering bills; tough residency restrictions for convicted sex offenders; and open space and other environmental bills.

So he created an exploratory committee for a Senate bid. But he fully expected a better-known candidate, like Rep. Carolyn McCarthy (D), a personal friend, or Rep. Steve Israel (D), an ally in Suffolk County politics, or Rep. Carolyn Maloney (D), whom he admires, to challenge Gillibrand. One by one, they announced they would not run.

While writer and labor activist Jonathan Tasini, who took 16 percent of the vote against then-Sen. Hillary Rodham Clinton (D-N.Y.) in the 2006 Democratic primary, has entered the race — he and Cooper met for five hours recently — Cooper believes he is now the only progressive with a chance to defeat the Senator next September.

“I absolutely understand that this is her race to lose,— Cooper said. “But I don’t think that Gillibrand should get a free ride. … I’ve met thousands of grass-roots Democrats at functions and Democratic clubs all over the place. The vast majority of them don’t trust her.—

So who is Jon Cooper, and why is he so confident?

Cooper, 54, grew up on Long Island. His family started Spectronics, a maker of ultraviolet lighting that he now runs, in 1955. It’s a union shop, with 165 employees and a 100,000-square-foot factory, one of the last big manufacturers left on Long Island.

Cooper and his partner, Rob — who legally changed his surname when the couple became parents — have five adopted children, ages 14 to 24. After holding a commitment ceremony in 2000, they were married in Old Greenwich, Conn., in April on their 29th anniversary.

In addition to his own business and political career, Cooper, who has a political science degree from Duke University, has become a major fundraiser for leading Democrats, including Clinton’s 2000 Senate campaign, Al Gore’s 2000 presidential campaign, and the 2004 presidential campaign of Sen. John Kerry (D-Mass.).

In 2007, when most New York pols were lining up behind Clinton’s White House bid, Cooper became the first elected official in the Empire State to endorse Obama, and he was quickly invited to Chicago to confer with top campaign strategists. Cooper was named Obama’s Long Island chairman, and he did outreach for the campaign throughout New York state with an array of ethnic and interest groups.

Cooper said he sees parallels between the 2008 presidential primary, when many Democratic insiders were urging Obama to stay on the sidelines and defer to Clinton in the name of party unity, and Gillibrand’s race.

“That’s the same argument that everybody’s using now,— he said.

By Cooper’s account, he raised in the “high six figures— for Obama — a number that would have topped $1 million, he said, but for a scheduled fundraiser on the night of one of the presidential debates that had to hastily be scrapped. So while he has never had to raise more than $85,000 for his own races, Cooper is confident he can ramp up his fundraising operation quickly if he decides to run. And he said he is also prepared to self-fund his Senate bid to a degree, though he would not say how much he is willing to invest.

“Sometimes,— Cooper said, “you’ve gotta tilt at those windmills.—