Powerful three-term Democratic Sen. Russ Feingold was defeated by Republican challenger Ron Johnson in his re-election bid Tuesday night.
Feingold, regarded by many as one of the most liberal members of the Senate, fell behind during much of the race.
Johnson, a businessman, challenged the incumbent senator on his support of President Barack Obama's health care law and for stimulus spending. His campaign struck a cord with voters tired of the sluggish Wisconsin economy.
Some analysts believe Johnson's win could foreshadow trouble for the president's re-election in 2012, after Obama won there in 2008.