Fitzgerald broke the news to supporters at a private luncheon at the Union League Club and told an afternoon news conference he would leave the Senate to spend more time with his wife and 10-year-old son, Jake.
"This is a race in a heavily Democratic state that for a Republican candidate will need full-time devotion in order to win," Fitzgerald said. He said he was unwilling to be a full-time senator and a part-time father.
The Chicago Tribune said the 42-year-old banking heir told close associates Monday night he had "no fire in the belly" for another campaign and was concerned about the cost of running for re-election. Fitzgerald spent about $13 million of his own fortune in a successful campaign to defeat Democrat Carol Mosley Braun, the first black woman to serve in the Senate, in 1998, becoming the first Republican to win a Senate race in Illinois in 20 years.
Staunchly independent, Fitzgerald bucked the state's traditional Republican establishment and had few supporters in top GOP circles near the end of his first-term, including House Speaker Dennis Hastert, R-Ill.
He was regarded as one of the most vulnerable incumbents in the Senate, where Republicans hold a slim 51-seat majority.
A poll released by Fitzgerald's office Friday said 56 percent of Republican voters asked said he deserved re-election, but nearly 25 percent weren't sure. The survey of 600 general election voters conducted by Hill Research Consultants of Houston found more than two-thirds of Republican voters and more than half of Illinois voters rated him favorably.
Members of the business community abandoned Fitzgerald because of his opposition to Chicago Mayor Richard M. Daley's pet project, expansion of O'Hare International Airport, and he alienated and angered party regulars by not working with fellow members.
While party insiders said Fitzgerald failed to bring home enough federal pork projects his legacy will probably be recommending three independent U.S. attorneys to fight corruption and cronyism in Illinois.
Former Illinois Attorney General Jim Ryan, who lost a bid for governor in November, is seen as a frontrunner for the nomination. Former Gov. Jim Edgar, former state Sen. Patrick O'Malley, businessman and former prisoner of war John Borling, Illinois Comptroller Judy Baar Topinka and former Lt. Gov. Corrine Wood are possible candidates in a wide-open primary.