A former New York City union official who challenged Hillary Rodham Clinton in the 2006 Democratic primary says he will again run in a primary — this time against incumbent Sen. Kristen E. Gillibrand.

“Unlike 2006 when I never really woke up each morning thinking I could win, I actually believe I can win this race,” Jonathan Tasini said. “The current senator is not elected by the voters and quite disliked by a large constituency for lots of reasons.”

Gillibrand, appointed by Gov. David A. Paterson to the vacancy created by Clinton’s departure to become U. S. secretary of state, has encountered some political difficulty from the liberal wing of the Democratic Party for her stand on gun control and other issues. Tasini believes he can appeal to those voters.

“If I can raise the money, it will be a significant race,” he said.

Though Rep. Steve Israel of Long Island recently said he would not challenge Gillibrand, at the request of President Obama, Reps. Carolyn Maloney of Manhattan and Carolyn McCarthy of Long Island are still weighing a primary race. Tasini said he welcomes everyone’s participation, even if more candidates dilute any anti-Gillibrand vote.

Tasini, who lives in Manhattan’s Washington Heights section, is the former president of the National Writers Union. Clinton beat him handily in the 2006 primary, though he did garner substantial support from left wing Democrats, including several groups in Buffalo.