1998 and 2010 were both relatively close races for Feingold and there has been alot of opinions floating around about why he won in '98 and lost in '10.
In a nutshell, Feingold had a better turnout and better margins in 2010 in his two strongholds: Milwaukee and Dane. The problem, though, was that while Feingold was eating his spinach and turning into Popeye in Dane and Milwaukee, the GOP was turning into the Incredible Hulk in the suburban areas accross the state-- most profoundly in the South East part of the state.
You need look no further than Waukesha. In 1998, the margin was 36,830, while in 2010 it was a monsterous 80,430 --actually a larger margin of victory than Milwaukee!
Below are the numbers for the 20 largest differences in victory margins between 1998 and 2010: