Among others who were lent money, Mr. Giannoulias’s opponents noted, was Antoin Rezko, a real estate developer and political fund-raiser whose ties to them  and whose conviction for fraud and bribery  have embarrassed more than a few Illinois politicians.

And the notion that Republicans may lump in Mr. Giannoulias as part of this state’s Democratic troubled political establishment, given the embarrassments involving Mr. Blagojevich (whose corruption trial will begin right in the middle of this campaign season) and others, worries some Democrats as they think ahead to November.

In an interview on Wednesday, Mr. Giannoulias said that he had not worked at his family’s bank for four years and that, like thousands of American family businesses, his was “not immune” to challenges created by “bad decisions in Washington, D.C., and lax oversight on Wall Street.”

“It’s telling,” Mr. Giannoulias said of Mr. Kirk, “that while I’m talking about ideas and creating jobs in Illinois, he’s focused on ridiculous political attacks. What people are looking for is someone who is talking about ideas.”

At a time when bankers and Illinois politicians hardly seem popular, Mr. Giannoulias has already begun portraying Mr. Kirk as a different stereotype that has faced backlash in recent elections  a Washington insider. Mr. Kirk, who is 50 and has served five terms in Congress, “is steeped in Washington, D.C., politics,” Mr. Giannoulias said.

Without doubt, political analysts here said, Mr. Kirk, like all Republicans, faces an uphill challenge in a general election in Illinois. Though each campaign claimed Wednesday that it had polls showing its candidate ahead, the clear trend over the past decade shows that more of the state’s 7.5 million registered voters consider themselves Democrats than Republicans. (Illinois voters do not formally register with a party.)

Mr. Kirk, who has favored abortion rights and cap-and-trade legislation intended to reduce carbon emissions, irks some of the most conservative Republicans in this state, including some who describe themselves as supporters of a Tea Party movement. He has since said he would oppose the cap-and-trade bill, a switch some saw as an effort to appeal more to conservatives. But political analysts believe his best chances of success may actually depend on him seizing independents and fed-up Democrats  segments that may have helped moderate Republicans, like James Edgar and James R. Thompson, former governors, win in this state years ago.