New York Gov. David Paterson (D) said Wednesday that he will officially declare a vacancy in the Empire State’s 20th Congressional district on Feb. 23, likely setting up a special election on March 31.

The upstate seat has been empty since late January, when Paterson tapped then-Rep. Kirsten Gillibrand (D) to replace now- Secretary of State Hillary Rodham Clinton in the Senate.

Both parties have already selected nominees for the special election, with state Assembly Minority Leader James Tedisco running for Republicans and venture capitalist Scott Murphy as the choice of Democrats. Murphy is formally kicking off his campaign at his headquarters in Glens Falls today.

Murphy will begin airing an introductory bio spot on Albany TV stations today, spotlighting his immediate family — and his wife’s large extended one. Tedisco is up with a Web video showing him announcement speech from earlier this month.

Republicans have a 70,000-voter edge in party enrollment and see the district as a major pickup opportunity. Surrogates for each candidate have already begun slinging mud at the other, and both parties are expected to play heavily in the race.