CHICAGO — The tally of a close Wisconsin Supreme Court election, which had come to be a referendum on Republican leadership in the state, turned upside down on Thursday evening: the incumbent justice, viewed as a conservative, took a lead of more than 7,000 votes after a clerk in one Republican-leaning county announced she had initially failed to report some 14,000 votes.

The development quieted expectations that the race would be decided in a statewide recount. But it also set off a wave of skepticism from Democrats and union supporters, who had viewed the contest as an outlet for their fury at the Republican cuts to collective bargaining rights. Those forces had supported the challenger, and said they found it convenient that votes for the incumbent, Justice David T. Prosser Jr., were suddenly discovered.

“Wisconsin voters as well as the Kloppenburg for Justice Committee deserve a full explanation of how and why these 14,000 votes from an entire city were missed,” said Melissa Mulliken, the campaign manager for JoAnne Kloppenburg. Ms. Kloppenburg declared victory over Justice Prosser on Wednesday when an unofficial vote count showed her with a 204-vote lead out of more than 1.4 million cast.

The margin had shifted slightly several times Thursday as local election officials began reviewing their counts, a process that may still change the margin in the days ahead. But the disclosure of overlooked votes from Waukesha County, a Republican, suburban stronghold near Milwaukee, was a dramatic swing.