State Senator Simcha Felder, the Democrat who has caucused with Republicans since his election in 2012, will join the Democratic conference for the first time, party leadership confirmed Monday.

Over the years, Felder has repeatedly stymied Democratic priorities, with votes against the New York Dream Act, speed cameras in school zones and a ban on plastic bags, measures which finally passed this year.

But on Monday, Senate Majority Leader Andrea Stewart-Cousins signaled she was ready to put all that behind her.

“This year, Senator Felder joined our Senate Democratic Majority in support of many crucial issues,” she said in a statement that pointed to Felder's support of new tenant protections, a bill allowing undocumented immigrants to get drivers licenses, gun control measures and voting reforms. “Accordingly, we will move forward with 40 diverse and united members joined by a shared commitment to continue delivering progressive results for our state.”

Felder released a terse statement, saying, "I look forward to working together with my Democratic colleagues on behalf of my constituents and all New Yorkers.”

He didn’t immediately return a request for an interview.

But Albany watchers were already criticizing his decision as opportunistic. Doug Muzzio, a professor of public affairs at Baruch College, said he was dismayed by the Senator’s apparent “ideological flexibility.”

“Political whoredom wins again," Muzzio said. "He sold out to the highest bidder. He sold out to the Republicans and now he’s selling out to the Democrats. He goes where the power is.“

He added: "If I were the Democratic leadership, I wouldn’t let him in.”

Michael Li, senior redistricting counsel at the Brennan Center for Justice at the NYU School of Law, said the upcoming redistricting after the 2020 census — which could rewrite the lines of Felder’s heavily Orthodox Jewish district — may be another reason why the state Senator might have felt pressured to join Democratic ranks now.

“I think it’s more playing along and being constructive in the next legislative session and voting on leadership’s priorities and the caucus’s priorities,” he said. “Then I think there is a greater likelihood they’ll work with him on preserving his seat.”

After nearly a decade as a City Councilman, Felder, who is an ordained rabbi, was elected to the State Senate as a Democrat, beating out the Republican incumbent in a new congressional district that conveniently connected Orthodox Jewish constituencies in Borough Park and Midwood. A few weeks after he was elected, he switched over to caucus with the Republican Party.

Felder’s constituents said they weren’t shocked by his choice to caucus with Republicans. He was simply repaying a favor for the newly drawn district that gave Orthodox Jewish constituencies a representative in the State Senate, according to Alexander Rapaport, an Orthodox community activist and Borough Park resident.

“It wasn’t like a ‘deal,’ it was more like the obvious,” he told Gothamist/WNYC.

Since that time Felder has run mostly unopposed, on the Democratic, Republican, Conservative and Independence Party lines, and occasionally running on all four party lines at once, like he did in 2018.

He gained outsized power last year when the Independent Democratic Conference — the group of rogue Democratic Senators who sided with Republicans — dissolved and another Republican State Senator left on active military duty, leaving Felder with the deciding vote that would give either party the majority. Last spring he was accused of hijacking the state budget process, when he refused to vote on it before getting certain carve-outs for state oversight of religious Jewish yeshivas.

“Most in the Orthodox community would think that he held up the budget too few times, rather than, “Wow he held up the budget once cause he needed something,’” Rapaport said.

But in 2018, Democrats flipped the Senate, giving them a decisive 39-vote majority to the Republicans’ 22 and Felder’s leverage in the Senate all but disappeared. Late last year he showed interest in returning to the Democratic conference but was rebuffed by leadership.

“He was basically ostracized,” said Susan Lener, Executive Director of Common Cause New York. “This is a clear recognition that the Democrats are in the majority and they are likely to stay in the majority.”

Felder’s constituents are unlikely to be daunted by his latest flip-flop, Rapaport said, because they know Felder will continue to represent Orthodox residents what is most important to them: yeshivas.

“That’s his only issue and that’s the overarching issue," Rapaport said. "They care less about the party he’s in. That’s the biggest kitchen table issue in every family including my own. Paying tuition [for yeshivas] is what we work for all our life.”