ALBANY, N.Y. - State Senate Majority Leader Dean Skelos, who is facing federal corruption charges, could see another challenge to his powerful position as soon as Monday, if growing opposition within his own Republican conference doesn't prompt a change sooner.

A Niagara County Republican said Thursday he would work to remove Skelos (R-Rockville Centre) on Monday if he doesn't step down earlier. Skelos fought back, issuing a statement listing his priorities for the rest of the legislative year. But other Republicans said it would be hard for Skelos to hold on past Monday.

"When the Senate returns to session, I will be supporting, with a heavy heart, a motion, or submitting one of my own if required, to replace Senator Skelos as majority leader," Sen. Robert Ortt (R-North Tonawanda) said on his Facebook page. Seven upstate Republican senators now have called for Skelos to step down as leader after federal officials charged him and his son with extortion, soliciting bribes and conspiracy.

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Sen. John DeFrancisco (R-Syracuse) and Sen. John Flanagan (R-East Northport) are seen as leading contenders to possibly replace Skelos, the leader since 2011.

Lawmakers are slated to return to the State Capitol on Monday. Democrats, who unsuccessfully tried to force a no-confidence vote on Skelos Wednesday, indicated they might do so again, emboldened by Republican critics of Skelos.

"We are going to continue to make our point that he must step down or be removed as leader and we will use all necessary means both on and off the floor of the Senate to accomplish that goal," said Mike Murphy, spokesman for the Senate's Democratic conference.

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The Senate's Republican majority conference is scheduled for another closed-door session Monday afternoon. But the Republicans could meet or discuss the issue before then.

"Monday could be D-Day for Dean," said one Republican senator, who noted that pressure is increasing from constituents and advocacy groups.

"It is clear the votes are not there and that he has lost confidence with the conference -- that's clear," said another Republican close to Skelos. "It's just a matter of time for him to accept it and take the next step."

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"Monday, at the far end," said another Republican close to senators, when asked if Skelos could survive as leader.

Federal officials have alleged that the senator compelled a major real estate developer and an environmental company to hire his son Adam and send him title-insurance and consulting work, amounting to more than $200,000 in the past four years. Dean Skelos has said he and his son are innocent.

A "consensus" of the 33 Republican senators in a closed-door meeting on Monday supported Skelos. His support seemed to be fleeting: Late on Wednesday Skelos issued a statement listing 16 supporters.

Skelos didn't comment Thursday, but continued to try to operate as if it were business as usual. He released an unprompted press statement that listed his accomplishments as well as the important issues he intends to tackle before the end of the session on June 17.

"While the state's fifth consecutive on-time budget provided us with a great start to this year's session, we must now finish the job -- for businesses, for taxpayers and for the hardworking residents of this great state," Skelos stated. Later, he attended a Senate Republican Campaign Committee fundraiser in Manhattan.Sen. Kathy Marchione (R-Halfmoon)Thursdaybecame the seventh Republican to abandon Skelos. "My calling for Senator Skelos to step down was necessary because it had become clear that he cannot effectively lead our conference," she said.

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The sixth, Sen. Betty Little (R-Queensbury), told the Glens Falls Post-Star: "It's unfortunate, but it's just not going to work this way."

Opposition to Skelos also is rising from some county GOP chairmen, the Conservative Party and gun-owner groups that have influence upstate.

Tom King, head of the New York State Rifle and Pistol Association, urged his members to support DeFrancisco, citing his opposition to the SAFE Act, which Flanagan supported.

The Republican close to Skelos said he believed it is a "done deal" for DeFrancisco to take over as leader, after some quiet campaigning by candidates for the job, although others said the debate was ongoing. The Republican said Flanagan, the Senate Education Committee chairman, was seen as too deeply connected to the Common Core effort that has enraged some parents and teachers statewide. Others said some members feel "it's upstate's turn" to hold the office.