Wisconsin Supreme Court Judge David Prosser found himself in an unexpectedly heated race after Gov. Scott Walker signed a bill limiting unions in the state. ((AP Photo/The Eau Claire Leader-Telegram, Paul M. Walsh))

No one was paying much attention to the campaign or to his opponent JoAnne Kloppenburg , an assistant attorney general and longtime environmental lawyer.

All that changed when Gov. Scott Walker (R) introduced his budget bill, sparking a bitter battle on all levels of state government over some collective bargaining provisions aimed at organized labor.

“You could feel the energy of this race change almost instantly,” said Brian Nemoir, Prosser’s campaign manager of that moment.

Today’s vote then is being looked at by opponents and proponents of the Walker bill, which passed the state legislature last month, as an early test of the political fallout from the fight. (There’s another election today that’s a referendum of sorts on Walker — the race for Milwaukee county executive.)

As Democrats work to recall Republican legislators who voted for Walker’s proposal, the legislation is headed to the state’s highest court. Right now, that court leans conservative with a bare four to three majority. If Kloppenburg is elected, that majority would shift to the ideological left.

“Before this campaign began, David Prosser was polling ahead of JoAnne Kloppenburg,” said Stephanie Taylor of the Progressive Change Campaign Committee, a group that’s been heavily involved in the race. “So if Prosser loses today, this will signal a real rejection of Republican policies by Wisconsin voters.” But labor advocates working in the state say no one knows what will happen today.

Despite the heightened interest, state officials are predicting turnout will be as low as it usually is in spring elections — about 20 percent. But in the Democratic strongholds of Dane and Milwaukee counties, city clerks are saying they expect turnout closer to 60 percent. What little polling has been done suggests the race is very close. (Wisconsin Supreme Court elections are publicly financed, meaning the campaigns don’t have much money for polling operations).

“The opposition has certainly been more visible,” said Nemoir. “But the reality will be better defined by today’s ballot box.”

Prosser has been on the court for 12 years; he was appointed to fill a vacancy by then-governor Tommy Thompson (R) in 1998. In 2001, he was up for election, but his was the only name on the ballot. So this is his first real race. While he calls himself a “judicial conservative” on his website, the judge has tried to distance himself a bit from Walker. In December, his campaign manager sent out a press release saying the judge would be a “complement” to the new governor and Republican-held legislature. Prosser said he wouldn’t have chosen that word. Democrats, of course, are happy to use it in their attack ads.

Those ads have been coming fast and furious; outside spending on the race could top $5 million. Supreme Court races in Wisconsin are technically non-partisan — candidates are chosen in a “top two” open primary with no party affliation on the ballot — but that doesn’t bar political parties from involvement.

As of last Sunday, pro-Republican special interest groups had spent about $1.9 million on ads, while Democratic groups had spent $1.2 million. Tea Party Express and the state Club for Growth are among the groups pouring money into the race for Prosser. Most of the money spent against Prosser comes from the Greater Wisconsin Committee, which has been running hard-hitting, sometimes controversial ads claiming the judge used sexist slurs and protected a pedophile. There are robocalls on both sides; one today features a Democratic state senator who fled the state during the budget negotiations. Progressive activists are calling tens of thousands of Wisconsin voters, reminding them to vote. Last week, a former Democratic governor who was Prosser’s campaign co-chair resigned and endorsed Kloppenburg. Even former Alaska governor Sarah Palin has weighed in.

If Kloppenburg takes Prosser down, it will be read as a major rebuke for Walker. If Prosser wins, it will be a sign that populist anger may not translate directly to the ballot box. Regardless of the outcome, he wouldn’t actually step down until August — meaning he could rule on Walker’s collective-bargaining legislation as a lame-duck judge who just lost his seat.