A state senator defeated the widow of Democratic Rep. George E. Brown Jr. in a special election today for her husband's seat, advancing to a Nov. 16 runoff against a Republican businessman.

The race is seen as an opening act of the 2000 election. Democrats dominated the primary, which Republicans had seen as a test of their ability to build on the party's 222 to 211 seat majority in the House.

With all precincts reporting, Joe Baca, a moderate state senator, had 31.5 percent. Fellow Democrat Marta Macias Brown had 30.2 percent, and Republican businessman Elia Pirozzi had 27.5 percent.

Seven other candidates trailed.

Gun control emerged as the key national issue in the campaign, particularly in the race between Baca and Marta Macias Brown.

George Brown, a liberal and longtime booster of science spending, died in July halfway through his 18th term.

His wife had emerged as a staunch advocate of banning assault weapons and cheap handguns.

She accused Baca of being "the radical gun lobby's favorite Democrat," noting that he abstained from a recent vote to ban the production and sale of cheap pistols known as Saturday night specials and took campaign contributions from gun lobbyists.

Baca, 52, responded with a mailer that pictured him standing next to Gov. Gray Davis as the governor signed a bill requiring that guns be sold with trigger locks.

It also noted that he voted to ban unaccompanied minors from gun shows.

Brown and Baca are both Hispanic and courted Hispanic residents, who represent more than a fourth of the district's 244,000 registered voters.