As the House on Tuesday prepared to swear in one new member, Democrat Laura Richardson of California, Massachusetts voters were taking the first step toward choosing another.

Bay State voters were choosing nominees for an Oct. 16 special election to replace nine-term Rep. Martin T. Meehan, D-Mass., who vacated his 5th District seat July 1 to become a chancellor of the University of Massachusetts campus in his hometown of Lowell.

Polls show Democrat Niki Tsongas is the frontrunner in the 5th District, which typically votes Democratic in federal elections. Tsongas played a visible role in the political career of her late husband, Paul Tsongas, who represented most of the current district in the House from 1975 through 1978, when he was elected to the Senate. He sought the Democratic presidential nomination in 1992, and died of cancer in 1997.

A Survey USA poll for WBZ-TV in Boston dated Aug. 30 showed Tsongas, dean of Middlesex Community College, leading with 40 percent to 29 percent for Eileen Donoghue, a Lowell city council member. Three state representatives trailed far behind, with 15 percent for Jamie Eldridge, 9 percent for Barry Finegold and 3 percent for James Miceli.

On the Republican side, with Jim Ogonowski, a hay farmer and retired Air Force officer, was leading Tom Tierney, a self-employed actuary, by 72 percent to 12 percent. Tierney was the 2004 Republican nominee against Meehan — a race he lost by a ratio of more than 2-to-1.

Democrats hold both Senate seats and all 10 House seats in Massachusetts, and are solidly favored to keep the 5th District seat.