Suffolk Legis. Jon Cooper was ready to make a primary challenge to Sen. Kirsten Gillibrand, until he met her, he said Monday.

Cooper (D-Lloyd Harbor) ended his efforts to run for the U.S. Senate Monday, throwing his support to Gillibrand (D-N.Y.) at a Mineola news conference where the two pols - whose camps spent much of the summer and fall sniping at each other - did everything short of singing "Kumbaya."

Cooper, 54, said he decided to pass on the race next November because, during a dinner last month in Washington, D.C., Gillibrand convinced him that she is a "true progressive."

"She is a woman quite unlike the one who was portrayed in the press," Cooper said. "My one regret is that I didn't sit down with Sen. Gillibrand three or four months ago."

Gillibrand, appointed in January by Gov. David A. Paterson to fill Hillary Rodham Clinton's seat, returned the compliments, calling Cooper "a man who has been leading the fight for all of us."

It was a sharp turn for Cooper, who launched an exploratory committee in April, though his Senate campaign consisted of making a few trips upstate, sending the occasional anti-Gillibrand e-mail and raising about $50,000. Cooper said he was "98 to 99 percent sure" before the fateful dinner that he'd enter the race.

Unable to seek re-election to the Suffolk Legislature in 2011 because of term limits, Cooper has been approached by state Democrats interested in challenging state Sen. Carl Marcellino (R-Syosset) in 2010.

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State and Nassau Democratic chairman Jay Jacobs said Cooper is one of the party's top choices for the race. Cooper said Monday that he is flattered by the request, but is not thinking about making a state Senate run.

Married in April in Connecticut to his longtime partner, Robert Cooper, Jon Cooper won re-election in November to a sixth term in the county legislature.

Since taking office, Gillibrand has been saddled with poor poll numbers and low name recognition, though she has drawn support from state and national Democrats. Reps. Carolyn McCarthy (D-Mineola), Carolyn Maloney (D-Manhattan) and Peter King (R-Seaford) have each backed off challenging Gillibrand after initially showing interest.

Jonathan Tasini, a Manhattan union activist who is a Democrat, is Gillibrand's only announced opponent.