As voters prepare to head to the polls for tomorrow’s special election, the race for a vacant state Assembly seat on the Upper West Side has shaped into a battle between a long-time Democratic Party insider and her well-funded challenger.

Linda Rosenthal, a former aide to Rep. Jerrold Nadler, is running on the Democratic line, having secured the nomination last month with the backing of Mr. Nadler and the Manhattan president, Scott Stringer. Mr. Stringer held the seat in the 67th District until he assumed the borough presidency on January 1.

Ms. Rosenthal won endorsements from nearly every leading state and local Democrat, including New York’s two senators and the state attorney general, Eliot Spitzer. But her strong party support has not deterred Charles Simon, an attorney and former Justice Department official who put more than $100,000 of his own money into the race.

Mr. Simon also raised more than $130,000 in outside contributions, including $3,400 from his father-in-law, a former NBC News anchor, Tom Brokaw, according to campaign finance records. As of mid-February, he had outspent Ms. Rosenthal by a three-to-one margin.

While Ms. Rosenthal touted her 25 years of experience as a community advocate and political aide, Mr. Simon made reforming Albany the focus of his campaign. Pointing to his status as an outsider, and with an eye toward his politically connected opponent, Mr. Simon said that if voters want to shake up Albany, they should elect someone who is “not beholden to the system we’re trying to change.”

Despite his spending advantage, Mr. Simon is seen as a heavy underdog, and party leaders said Ms. Rosenthal’s endorsements and her position on the ballot, under the Democratic line, will figure greatly in an election that included little time for campaigning and in which voter turnout is expected to be very light.

“I’m not sure how effective the money will be,” a district leader, Stuart Shorenstein, said.

Ms. Rosenthal said she was “confident” of victory, but acknowledged her financial disadvantage. She had a fundraiser scheduled for last night, after campaigning in the afternoon with the City Council speaker, Christine Quinn. “It’s hard when someone uses their own personal money,” she said, later adding, “I don’t think you should be able to buy any seats in government.”

Mr. Simon called the suggestion that he was trying to buy the election “outrageous” and said he needed to spend money to get his message out.

Two other candidates are also running in the election – a Republican, Emily Csendes, and a former schoolteacher who petitioned to get on the ballot, Michael Lupinacci.

Special elections to fill vacant Assembly seats are also being held in two other districts in the city where the Democratic nominee is expected to prevail easily. In the 74th District, on Manhattan’s East Side, Sylvia Friedman is running to succeed Steven Sanders, who retired after 28 years in the Assembly. She scored an upset victory over Mr. Sanders’s chief of staff in the party’s nominating convention last month.

In the 59th District in Brooklyn, Alan Maisel is the Democratic nominee. He served as chief of staff to Frank Seddio, who vacated his seat to take an appointment as a surrogate judge.

Polls are open tomorrow between 6 a.m. and 9 p.m.