More: "It's no mystery," said Rep. Thomas M. Davis III (R-Va.). "You have a very unhappy electorate, which is no surprise, with oil at $108 a barrel, stocks down a few thousand points, a war in Iraq with no end in sight and a president who is still very, very unpopular. He's just killed the Republican brand."

Presumptive Republican presidential nominee John McCain (Ariz.) helped Oberweis raise money, and the NRCC pumped more than $1.2 million into the district -- using more than 20 percent of its cash on hand -- to no avail.

"Even if it was mostly about Jim Oberweis, it's a terrible sign," Rothenberg said. "It adds to Democratic energy and further depresses the Republicans. And you can't dismiss the idea that there is an atmospheric advantage for the Democrats."

Barack Obama campaigned on behalf of Bill Foster, who took over Dennis Hastert's vacated seat.