Sen. Peter Fitzgerald’s stunning announcement that he will not seek a second term leaves Illinois Republicans looking for a candidate. That could end up being either good or terrible news for the GOP. It all depends on how circumstances develop.

Incumbents, even vulnerable ones, almost always start off with an advantage over other candidates: the proven ability to win a seat. But Fitzgerald defeated a seriously flawed Democratic incumbent (Carol Moseley Braun) in 1998, so he never demonstrated the political muscle that most incumbents can claim.

[IMGCAP(1)] During an interview three months ago, Fitzgerald acknowledged that he faced a tough re-election race, primarily because Illinois has been moving so appreciably in the Democrats’ direction.

But Fitzgerald also had problems with his fellow Republicans, particularly those in his own backyard. He liked to view himself as a maverick, and he appeared to model himself after Arizona Sen. John McCain (R).

Obviously, the eventual GOP nominee in the Land of Lincoln will not have the advantage of incumbency. But it now appears that the Senator’s other greatest asset, personal wealth, wasn’t that much of an asset either, since he apparently wasn’t willing to put a lot of his personal money into his re-election bid.

Democrats argue that vacancy increases their chances of winning the seat next year.

Well, maybe. But everything will depend on who becomes the GOP nominee.

Two Republicans top the early list: former Gov. Jim Edgar and former state Attorney General Jim Ryan.

Ryan, who has had two battles with cancer, won a hard-fought, three-way primary for governor in 2002. But he lost the general election to Democrat Rod Blagojevich, 52 percent to 45 percent, primarily because the outgoing governor, Republican George Ryan, was a political disaster — and an albatross around Jim Ryan’s neck.

Jim Ryan ran a very credible campaign but had to expend valuable resources in the primary. He trailed badly throughout the general election. While an obviously flawed Zogby poll conducted a few days before the election showed Ryan leading Blagojevich by 1 point (44 percent to 43 percent), a much more predictive Research 2000 survey conducted during the same time frame found the Democrat leading 53 percent to 42 percent, close to the final result. (I’ll refrain from any other comments about the two polls. Just read between the lines.)

Shortly after that race, Chicago Sun-Times reporter Steve Neal wrote, “Win or lose, Ryan said that this would be his last race.” If that’s the case, Jim Ryan is out of the mix for the Senate seat.

Edgar, on the other hand, walked away from Illinois politics on his own terms — and at the top of his game. Indeed, his job approval stood at 72 percent when he opted not to seek a third term.

Edgar served as the state’s chief executive in times of economic plenty, and he left the state in relatively good overall shape. Like Ryan, however, he had medical issues while in office, including heart bypass surgery.

But don’t get ahead of yourself by assuming that Edgar is headed to Capitol Hill. Remember, the former governor was wooed by national Republican figures before, and he rejected overtures from then-Majority Leader Trent Lott (Miss.) and then-Republican National Committee Chairman Jim Nicholson to get into the 1998 Senate race against Braun. That’s how Fitzgerald won the Republican nomination and the seat.

Had Edgar run in 1998, he would have won the Senate seat easily. But he chose to walk away from elective office. Has Edgar, who is just 56 years old, changed his mind about wanting to go to Washington?

If you are about to conclude that Edgar will never run, consider this: the Senate now includes Elizabeth Dole (R-N.C.), Lamar Alexander (R-Tenn.) and Hillary Rodham Clinton (D-N.Y.). Would Jim Edgar be so out of place among that group?

If he runs, Edgar would be the 800-pound political gorilla in the race. A proven fundraiser, he would have appeal to the critically important moderate suburban voters in the “collar counties.” And he would almost certainly win White House support. You can almost see the wheels turning in Karl Rove’s head.

But Edgar wouldn’t be without his warts, no matter how small. During his time as governor, Edgar was embarrassed by a scandal involving employees taking bribes. That history would give Democrats ammunition and a way to tie him to the discredited George Ryan, whose administration suffered far more serious ethics problems.

In addition, conservatives might well resist an Edgar candidacy, since the former governor has always favored abortion rights and once proposed higher state taxes (in exchange for lower property taxes) to change the way education was funded in the state.

If Edgar decides against the race, state Treasurer Judy Barr Topinka, the only statewide elected Republican following the 2002 Democratic landslide, and wealthy former investment banker/inner-city school teacher Jack Ryan might consider the contest. Jack Ryan ruled out a run against an incumbent, but he might find the open seat too attractive to pass up. Few people now know who he is, but he could be a very strong candidate.

Fitzgerald’s announcement makes the Illinois Senate race even more interesting, because both parties now face uncertainty. In a few months, we may know whether the Senator’s decision hurt his party or actually helped the GOP.