New York teachers unions’ most powerful defense against Gov. Andrew Cuomo’s aggressive education agenda is now handcuffed.

Sheldon Silver, an experienced negotiator who has helmed the Democratic conference in the state Assembly for more than 20 years, was arrested on federal corruption charges Thursday. His arrest threw the state’s nascent legislative session into “chaos” and, at least temporarily, left the teachers union without its most vocal champion in Albany at a crucial moment.

Although the city’s union president sought to strike a business-as-usual tone in the aftermath of Silver’s arrest, other leaders weren’t as optimistic. Patrick Walsh, a teacher and union chapter leader at P.S. 149, wrote that Silver was “the one man in all of New York state with the political clout and will to at least mitigate Cuomo’s assault” and called his arrest a “catastrophe.”

The arrest came less than 24 hours after Cuomo delivered a fiery State of the State speech attacking New York’s public education system and announced he wants to expand charter schools, toughen teacher evaluations, and raise the bar that teachers must clear in order to receive tenure over the next few months. Teacher unions oppose those proposals and have counted on Silver’s influence to weaken or nix certain bills they are most opposed to.

Silver has enjoyed enormous support as Assembly Speaker among more than 100 Democratic colleagues who vote together on legislation, despite lingering corruption allegations. One of “three men in a room” for the final negotiations over the state’s budget and other legislation, Silver’s power, for better or worse, is a testament to the closed-door nature of New York state politics.

Silver’s main education-related goal has been to reduce school overcrowding, and last year he got Cuomo to allow New York City to spend money meant for technology on eliminating classroom trailers. But he has also stepped in to add extra regulations to legislation meant to expand charter schools.

When the charter-school cap was last lifted in 2010, Silver was instrumental in getting Mayor Bloomberg to concede to a ban on for-profit charter operators and to regulations on enrollment of special-needs students. Last year, a new law that guarantees facilities funding for New York City charter schools was passed only after Silver added a provision that allowed the city and state comptrollers to audit charter school finances.

“Leadership in the legislature requires an enormous amount of skill and Shelly has shown himself to have that skill,” James Tallon, a member of the Board of Regents and former Assembly member, said in a radio interview.

Silver faces bribery charges for accepting more than $4 million from real estate firms that did business with state government and which he failed to report in annual disclosure forms. Federal prosecutors had been investigating the alleged pay-to-play scheme since the summer.

Silver’s colleagues rallied in the immediate aftermath of his arrest this morning, saying they continue to support him in his role at Speaker of the Assembly, according to State of Politics.  And his union allies insisted that they’re not straying from their top priority of securing more funding for schools.

“We’re still moving forward,” said United Federation of Teachers President Michael Mulgrew. “We’ll continue to advocate for what works for children.”

Mayor Bill de Blasio, another Silver ally, called the charges “very serious” but called for patience.

“I’ve always known Shelly Silver to be a man of integrity, and he certainly has due process rights,” de Blasio said.

Even Silver’s political adversaries in the Assembly credited Silver for his leadership and expressed anxiety about what could happen if he resigned.

“My attitude is be careful what you ask for,” said Al Graf, a Long Island Republican and member of the education committee. “I’m not the biggest fan of Shelly Silver. We don’t agree on everything, but I do recognize the fact that when he was there, the trains were running. And now the train just ran off the track.”