ALBANY – Gov. Paterson, defying the liberal wing of his Democratic Party, has chosen little-known, NRA-backed, upstate Congresswoman Kirsten Gillibrand to succeed Hillary Rodham Clinton as New York’s junior senator, it was learned last night.

The surprising – and, for many Democrats shocking – decision to pick the conservative Gillibrand, 42, from Hudson in Columbia County, was disclosed by the governor in calls to party officials and some members of the state’s congressional delegation, many of whom said they were unhappy with the selection, sources said.

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Gillibrand, a mother of two occasionally resented by colleagues for being an aggressive self-promoter, was strongly backed for the post by Charles Schumer, the state’s senior senator, who said a woman and an upstater was needed on next year’s ticket.

Paterson’s decision – to be officially announced today at noon at the state Capitol – was made just 24 hours after Caroline Kennedy took herself out of the running.

The decision was a major rebuff to some of the state’s best-known Democrats interested in Clinton’s seat, including Attorney General and former federal Housing Secretary Andrew Cuomo, for whom Gillibrand once worked as a junior lawyer; Nassau County Executive Tom Suozzi, and Reps. Carolyn Maloney of Manhattan and Steve Israel of Suffolk County.

Sources said “at least five” members of the state’s Democratic congressional delegation called Paterson to protest the possibility of Gillibrand’s selection. One, Rep. Carolyn McCarthy of Nassau County, even threatened a primary challenge. Gillibrand faces a special election in 2010.

Democratic activists predicted that Cuomo, son of former Gov. Mario Cuomo, would also “seriously consider” challenging Paterson in a primary next year.

Calls were made from Paterson’s office to leading Democrats and Republicans throughout the afternoon, inviting them to a special meeting room attached to the Capitol for “a major announcement.”

The inclusion of several prominent Republicans among the invitees was an early sign to insiders that Paterson planned to pick an upstate Democrat.

Gillibrand has won two successive elections in one of the heaviest GOP districts in the state, first upsetting incumbent Rep. John Sweeney and, in November, defeating former state GOP chairman and multimillionaire Alexander Treadwell, in one of the most expensive races in the nation.

Liberal Democrats have been wary of her because she ran for re-election with the backing of the National Rifle Association, opposed the federal TARP program to rescue banks, and has been less than enthusiastic about gay marriage.

fredric.dicker@nypost.com