First-term Sen. Peter G. Fitzgerald (R-Ill.), widely regarded as the most vulnerable Senate incumbent facing the voters next year, announced yesterday that he will not seek reelection, pushing the Illinois race to the forefront of battles for control of the narrowly divided chamber.

Democrats immediately claimed that Fitzgerald's decision boosts their prospects to pick up the seat, but some strategists of both parties suggested that the GOP might benefit from a fresh face without the scars that Fitzgerald bore from fights with party colleagues.

Republican fortunes could soar if former governor Jim Edgar, a popular figure who stepped down in 1999 after two terms as the state's chief executive, decides to run, several GOP strategists said. Edgar would be the "instant front-runner," said Bob Kjellander, Republican national committeeman from Illinois. A GOP source said Edgar appears to be "giving it real serious thought."

At a news conference in Chicago, Fitzgerald said he wants to spend more time with his wife and 10-year-old son and did not like the idea of spending the rest of his term in full-time campaigning.

But others said he also appeared to have been swayed by discouraging poll numbers, the likelihood of a primary challenge, the declining strength of the GOP in Illinois and the enormous likely costs of the campaign, much of which he might have had to absorb himself. Also, a fellow Republican said, "the old Fitzgerald enthusiasm just didn't seem to be there anymore."

Democratic strategist David Axelrod said, "He was just roadkill in the eyes of many politicians."

Fitzgerald, 42, an heir to a Chicago-area banking fortune, pursued a conservative but often independent course in the Senate and repeatedly tangled with other Illinois Republicans, including House Speaker J. Dennis Hastert, who regarded him as less than a team player.

Colleagues accused Fitzgerald of not supporting local projects, and he created a furor in both parties when he blocked legislation clearing the way for expansion of Chicago's O'Hare International Airport.

Fitzgerald was a little-known state senator when he narrowly defeated Sen. Carol Moseley Braun, the first African American woman to serve in the Senate, who had run into reelection troubles of her own. He spent nearly $13 million of his own fortune on the race, a feat that could make it hard for him to collect contributions for a second campaign.

Fitzgerald is the second Senate incumbent to announce he will not seek reelection next year. The first was Zell Miller (D-Ga.), a conservative who often votes with the GOP. Republicans, who won Georgia's other Senate seat last year, concede that Miller could probably have won reelection but figure they have a good chance of picking up the seat without him in the race.

The Senate has 51 Republicans, 48 Democrats and one independent who aligns with the Democrats.

In addition to Edgar, Republicans mentioned as possible candidates included former Illinois attorney general Jim Ryan, state treasurer and Republican Party chairman Judy Baar Topinka, retired investment broker Jack Ryan, former lieutenant governor Corinne Wood and Andrew McKenna, a businessman who planned to oppose Fitzgerald in the GOP primary.

Possible Democratic candidates include state comptroller Dan Hynes, state Sen. Barack Obama; former Chicago school board president Gery Chico, wealthy businessman Blair Hull and Cook County Treasurer Maria Pappas.

Sen. Peter G. Fitzgerald (R-Ill.) announces his decision yesterday.