Now that it has officially endorsed a primary opponent against him (Gustavo Rivera), the Working Families Party is stepping up its effort to oust Senate Majority Leader Pedro Espada Jr., sending out an on-line fundraising appeal that deems the controversial Bronx lawmaker “the worst politician in Albany.”

“State Senator Pedro Espada has repeatedly blocked progress for working New Yorkers. He’s used taxpayer money for his own enrichment and switched political parties to kill pro-tenant legislation, shut down the state government, and increase his own power,” WFP Executive Director Dan Cantor wrote.

“…Espada’s not going down without a fight. He’s funded by Albany lobbyists and big real estate interests, and they’ll spend serious cash to keep him in power.”

“But Espada’s constituents in the Bronx are sick of him siding with landlords over tenants and diverting their tax dollars to fund his personal schemes. Espada doesn’t even live in his own district, preferring a mansion in the suburbs.”

“Our candidate, Gustavo Rivera, is a true community leader. He’s spent years as an educator, organizer and progressive political leader. Putting him in office will be a huge step toward making Albany work for us.”

“There’s no doubt about it: if we want living wage jobs, stronger hospitals and schools, more affordable rents, lower property taxes, a cleaner environment, and a state government that actually gets things done, politicians like Pedro Espada have got to go.”

Cantor says the WFP is seeking to seed its grassroots effort against Espada with contributions of $5 apiece from 1,000 New Yorkers. According to WFP spokesman Dan Levitan, some $4,500 has been raised since the e-mail was sent Saturday morning.

The money raised is actually going into the WFP’s general campaign account and will help with its push to elect a “progressive Senate Democratic majority,” Levitan said. But the prty’s main focus will be Espada.

At the moment, there are three Democrats battling against Espada in the September primary. The fourth, Desiree Pilgrim-Hunter, dropped out last week, citing an inability to raise campaign cash as a main reason behind her decision.

Rivera has built up a considerable amount of support, including from Councilman Oliver Koppell, who endorsed the candidate today, and Bill Samuels’ PAC, the New Roosevelt Initiative.

This all puts the Bronx Democratic Party in a bit of a sticky situation, since one of the original Rainbow Rebels who supported the organization’s chairman, Carl Heastie, in his successful effort to oust Assemblyman Jose Rivera was Sen. Ruben Diaz Sr., who is Espada’s most outspoken challenger defender.

No wonder Assemblyman Jeff Dinowitz, chair of the party committee and a Rainbow Rebel himself, wasn’t in any big rush to heed the call of state Democratic Party leaders’ to start proceedings to dis-enroll Espada.

(Recall that Koppell was on the outs with the party after he initially sided with Rivera against Heastie and also turned his back on then-NYC Comptroller Bill Thompson to endorse Mayor Bloomberg last fall).

The push for Gustavo Rivera also puts Senate Democratic Conference Leader John Sampson in a bind. Sampson, who recently called Espada a “friend” and insisted he trusts him despite his repeated past involvement with the Senate GOP, might need the senator’s vote to pass the outstanding final piece of the budget – assuming the Senate ever returns to Albany, which isn’t looking likely so far this week.