We can't know what the result of this race would be, though I reckon Obama wins by a slim margin, assuming he is the nominee.
But that's not really the issue. As Glass Onion points out the issue is that there's no way the Kerry campaign is going to give the keynote address at the 2004 DNC to a state senator in a tight federal race who may look doomed by election day. Obama was chosen IOTL precisely because he couldn't lose.
To make an anology that shows what I'm getting at--if the Rubio/Crist senate race had occurred in 2008 would McCain have invited Marco Rubio to be his keynote speaker? Actually, I guess McCain was pretty desperate enough to try that, but you know what I mean.
Not being the star in Boston in 2004 could make it impossible for Obama to even think of running in the next presidential election, even if he does win his senate seat by a healthy margin.
A
pro-choice moderate.
The biggest problem with Edgar getting the nomination may be Karl Rove's desire to build a permanent conservative Republican majority.
I don't think Rove in 2003 (when the race was shaping up) wanted social liberal GOPers running for US Senate in states he thought Bush should be competitive in. If Illinois wasn't on his wish-list for becoming a Red State I'd be surprised.
What I've read suggests that the Bush Administration was trying to get Edgar to run, which suggests that on some level elements of the administration would find A Senator Edgar acceptable. Of course, Rove may have had some reservations, but given the fact that the Administration was asking Edgar to run, I can't see them pulling a complete 180 and attempting to block his election, or his nomination. Things didn't look good for Republicans in Illinois back in 2004. It was only two years after Ryan had to leave because of what he had done while Secretary of State. (George Ryan to the best of my knowledge is
not in prison for anything that occurred between 1999 and 2003, or in other words while he was actually Governor.) The administration had a good reason for backing Edgar, he arguably had the best chance of overcoming the post George Ryan issues that faced the GOP at the time.
In terms of Cook County to be rather cliche, Obama would have an advantage within the city itself, and Edgar would have the advantage in the suburbs.
Edgar might have a slight advantage in that he's a much more known quality than Obama was back in 2004. Obama was known relatively well in his state senate district obviously. But as state senator, he really hadn't done much of anything to distinguish himself despite latter campaign claims to the contrary.(For the record, I largely support him as President.) Edgar has statewide name recognition, Obama really doesn't. Obama would have to make an effort just to make himself known throughout the state. Because of the name recognition factor, an Obama victory would be considered an upset, regardless of Obama's advantages.
Given that Obama carries Chicago, and Edgar carries the suburbs, the election hedges on the more conservative rest of Illinois. From a Dupage County perspective, both Edgar and Obama are bad choices. Edgar's a moderate and Obama's a liberal. Simply because he's a Republican Edgar might do better than Obama there.
Edgar's real problem is the Ryan issue, but his popularity might allow him to overcome that. That's what the whole election hinges on here. Obama doesn't have most of his other advantages. Edgar isn't going to start sending campaign staffers to follow Obama around with a camera. Edgar doesn't have a nasty divorce record which would push him out of the race. And Edgar certainly isn't as far right as Alan Keyes. Edgar's surname is not Ryan. If Edgar loses, it's because Obama successfully linked Edgar to Ryan's crimes. Even then, he won't win by a "healthy margin" It'd be a very very close race IMO.
Obama's victory would probably come as an upset even from the perspective of the democratic establishment. People like Madigan would probably expect Edgar to win, and plan accordingly. After all, it was a Republican seat back in 2004. While Edgar's victory might be a set back for the Democratic party, it isn't something they'd be too concerned with.