With three members of the state Assembly having just vacated their seats representing Manhattan and Brooklyn, possible replacements are lining up to take their spots.

The seats belonging to Assemblymen Steven Sanders, Scott Stringer, and Frank Seddio became vacant as of January 1st and are anticipated to go to political insiders who work for elected officials in the city.

Unlike regular elections, the candidates in these races will be appointed by their parties’ county committees, after which the public will vote in a special election expected to be held in February.

Competition appears to be most stiff in the battle to succeed Mr. Stringer, who was elected in November to serve as president of Manhattan. Of the seven known candidates vying for the West Side post, the director of Rep. Jerrold Nadler’s Manhattan office, Linda Rosenthal, is considered the top contender. She is the only woman in the race.

Mr. Nadler himself held the 67th Assembly District seat for 16 years until he was elected to Congress in 1992. One of his political aides, Mr. Stringer, then won the spot in a special election. If elected, Ms. Rosenthal would continue that legacy.

Mr. Nadler vacated the seat to replace Rep. Ted Weiss, whose son, Thomas, is now one of the candidates gunning for Mr. Stringer’s seat.

Ms. Rosenthal, 48, has worked in Mr. Nadler’s office for 13 years.

Because the districts are heavily Democratic, whoever is put on the ballot by the county Democratic Committee is likely to take the seat.

In a surprise move late last year, Mr. Sanders announced he would step down from the 78th Assembly District, stretching from the Lower East Side up to Turtle Bay, after 28 years in Albany.

Among the three candidates competing for the spot is Mr. Sanders’s longtime chief of staff, Steve Kaufman, whom Mr. Sanders has endorsed. Mr. Kaufman, 50, who is gay and said he would actively fight to legalize gay marriage, has worked for Mr. Sanders for about 25 years. He said he was “involved with virtually every decision made.” A resident of Stuyvesant Town, Mr. Kaufman attended New York University and Cardozo Law School.

An attorney in private practice on Park Avenue, Donald Tobias, is also aiming to fill the seat. The former district leader who served on Community Board 5 said he believes the Assembly needs “new vigor, new energy, and new people.” Also running is Sylvia Friedman of the Gramercy Stuyvesant Independent Democrats club.

In Brooklyn, Mr. Seddio has vacated his seat in the 59th Assembly District in southern Brooklyn after he was selected by the party’s embattled boss, Clarence Norman, to take a newly created surrogate judgeship.

The county committee is expected to appoint Mr. Seddio’s chief of staff, Alan Maisel, to replace him. A retired public school principal, Mr. Maisel, 60, joined the assemblyman’s staff in 2001.

He said education would be his top priority and questioned whether the mayor’s initiative to take control of city schools is really working.

Mr. Seddio was elected in 1998 when Assemblyman Anthony Genovesi died in a car accident.

The three special elections are among the seven to take place across the state. If Governor Pataki calls a special election this month, it would be held in February.

In Manhattan a committee of 239 members will vote for whom to put on the ballot; in Brooklyn that committee is made up of 325 members.

Whomever is elected will still have to run for the seat again in the fall and is likely to face a primary. In an odd coincidence, all three of the outgoing members won their seats in special elections.

“Based on past history, even a short incumbency of a few months is a tremendous advantage,” a Democratic political consultant, Jerry Skurnik, said. “Candidates who have been elected this way almost always stay in office.”