Suffolk county legislator Jon Cooper has been mulling whether to challenge Kirsten Gillibrand in a Democratic primary next year, with an announcement set for Monday. And as he makes his final decision, party officials have been encouraging him to run for Senate -- State Senate, that is.

According to Democratic officials, Cooper is seen as one of the strongest potential challengers to State Sen. Carl Marcellino, a Republican whose district is split between Nassau and Suffolk. GOP enrollment there is still higher than the number of registered Democrats, but Cooper, who represents Huntington, could cut into Marcellino’s Republican base in the Suffolk part of the district.

“If you look at the enrollment in Marcellino's district, he makes up for very close Nassau enrollment by heavily Republican Suffolk enrollment,” said a Senate Democratic official. “If Jon Cooper can bite into that, while at the same time maybe rally the equality groups, the gay rights groups over marriage and things like that, that would make an interesting race.”

In an interview Wednesday, Cooper confirmed that he has spoken with Senate Democratic officials about challenging Marcellino as recently as this week, but said his focus remains on exploring a possible U.S. Senate bid.

“In recent weeks, a number of Democratic elected officials and Democratic Party leaders have urged me to consider running for New York State Senate against Senator Marcellino,” he said. “My answer has always been the same: that I did not have interest in running for that particular office at this time.”

He added: “But I guess the emphasis would be on, ‘at this time.’”

Cooper has been lobbied to consider challenging Marcellino in years past, including by then-Senate Minority Leader David Paterson. He demurred, thinking that he would be powerless in the Senate minority. Now, Democratic leaders believe, there may be more motivation for Cooper to run.

Recruiting him into a State Senate race would help clear the field for Gillibrand, who has gone to great lengths to shore up her progressive credentials. While most Democratic officials do not see Cooper as a serious threat to Gillibrand, they would nonetheless prefer that she run unopposed. (Labor activist Jonathan Tasini has already declared his intention to run.)

Running a strong candidate against Marcellino would also allow Senate Democrats to draw Republican resources away from their most vulnerable incumbents, including another Suffolk senator, Brian Foley. And Cooper would likely be a reliable vote in the senate on core progressive issues, such as same-sex marriage.

Cooper would not say what his plans are, but indicated that running for State Senate is something that he would consider, depending on what he announces Monday.

“Part of my thought process for a future potential run for New York State Senate would be, once again, what’s best for the Democratic Party and New York State,” he said. “Right now, if anything, I’m running for the U.S. Senate.”