In a semi-surprise move that could dramatically change the chamber’s calculus, Queens Sen. Tony Avella has decided to abandon his fellow so-called “regular” Democrats and join forces with Sen. Jeff Klein’s IDC.

Avella hasn’t yet formally announced his switch, and apparently hadn’t yet informed Senate Democratic Leader Andrea Stewart-Cousins et al when he confirmed his plan to the Daily News, saying the IDC “has developed a clear, progressive agenda for New York’s working families.”

Senate Democratic spokesman Mike Murphy issued the following statement this morning: “It’s unfortunate that progressive policies continue to be stymied because of divisions created by Senators who choose to empower Republicans.”

There were no obvious signs of Avella’s plans to defect. In fact, as recently as last summer, he had been critical of the IDC, saying (in his usual blunt way) the conference should “put up or shut up” when it came to a fracking moratorium – one of his top agenda items.

But Avella has always had an independent streak, and been something of a maverick. In 2009, he ran a long-shot primary challenge against the Democratic NYC mayoral favorite, Bill Thompson, losing in a landslide.

Last year, he ran a short-lived campaign for Queens borough president, and ended up pulling out of the race (eventually won by his former NYC Council colleague, Melinda Katz), saying he could “best serve the people of Queens” by remaining a senator.

Certainly – at least for the short term – being an IDC member will help Avella get his bills to the floor, thanks to the conference’s power-sharing agreement with the Senate Republicans.

But in the long run, the Democrats seem to have momentum on their side when it comes to taking back the majority, if not this year (probably not), then perhaps in 2016 when the next presidential election boosts turnout in this blue state.

For now, however, Avella’s decision to jump ship is a blow to the Senate Democrats, and in particular to his fellow Queens lawmaker, Mike Gianaris, who heads the DSCC and worked hard in 2010 to assist Avella in his successful challenge to former GOP Sen. Frank Padavan.

As the DN’s Ken Lovett points out, without Avella, the regular Democrats have just 24 members. Two Democrats indicted on corruption charges – Sen. Malcolm Smith (also of Queens, and a former IDCer) and Sen. John Sampson, of Brooklyn – are men without caucuses. Sen. Simcha Felder, of Brooklyn, sits with the 29 Republicans.

There are also two vacancies in the Senate – one on the Republican side, thanks to the New Year’s Eve departure of Chuck Fuschillo for the private sector, and the other on the Democratic side, following Eric Adams’ election to the Brooklyn borough president’s office.