Until Walker introduced his bill severely limiting collective bargaining for 175,000 public employees, Prosser was seen as a clear favorite. He got 55 percent of the vote in the four-way Feb. 15 primary to Kloppenburg's 25 percent.

"Prosser won Milwaukee city and county in the primary, so that needs to turn around — and turn around big-time — for Kloppenburg to have a shot," said Charles Franklin, a UW-Madison professor of political science.

Franklin said the heavier than normal turnout favored Kloppenburg because it meant voters who sat out the primary were energized to vote in Tuesday's election.

"Prosser would have sailed to re-election if this (controversy) hadn't come up," Franklin said. "I think a lot of the race has become a proxy fight between Walker and the unions."

The close results suggest the race may be decided by a recount. Under Wisconsin election law, a candidate has three days after the official results have been tallied to request a recount. The candidate must specify a reason for the request, such as a belief a mistake was made in the counting or some other irregularity.

Prosser has been on the high court for 12 years. He was appointed by then-Gov. Tommy Thompson in 1998 and elected to a 10-year term with no opponent in 2001.