A hand-painted sign highlighting Wisconsin’s April 5 state Supreme Court election declares: “This May Be the Most Important Vote You Ever Cast.”

That’s a bold claim.

But it won’t be dismissed by many observers of the six-week long struggle between Governor Scott Walker and the public-employee unions he seeks to dismantle with aggressive anti-labor legislation and tactics.

The Supreme Court election in Wisconsin—one of a number of Midwestern states that elect jurists, in keeping with the progressive tradition that said all powerful officials should be accountable to the people—will provide the first real measure of the strength of the mass movements that have developed to challenge Walker, his agenda and his political allies.

The Wisconsin Supreme Court currently has a 4-3 conservative majority that is expected to be sympathetic to Walker’s agenda as it faces extended litigation. But one of the conservative justices is facing an unexpectedly hard re-election fight. If he loses, the balance on the court will tip toward a majority that is more likely to check and balance the governor who has emerged as the authoritarian face of the national push by conservatives to break public-sector unions.

As such, the Wisconsin race is being watched closely by the governor’s critics—who have taken to calling the state “Fitzwalkerstan,” a combination of the governor’s name and that of his legislative consigliere, Senate majority leader Scott Fitzgerald—and by Walker’s allies in corporate boardrooms and right-wing think tanks far from Wisconsin.

On Friday, Sarah Palin endorsed the conservative incumbent, Justice David Prosser, who has also benefitted from expensive television advertising campaign’s funded by corporate donors.

But Prosser, a veteran political player and jurist who was expected until just a few weeks ago to coast to victory, appears to be in serious political trouble—at least in part because he has aligned himself so closely with the controversial governor and the Republican right. On Thursday, his campaign co-chair, a popular former governor, abruptly quit the campaign and endorsed Prosser’s challenger.

That’s unprecedented. But so, too, is Prosser’s determination to politicize what is supposed to be a nonpartisan judicial position.

Prosser has departed from the state’s best judicial values and traditions to identify himself as a conservative who will make decisions based on his political ideology and his political associations—particularly his association with Governor Walker—rather than the law.

His opponent, veteran Assistant Attorney General JoAnne Kloppenburg, has done the opposite, positioning herself as a rule-of-law contender who would serve as an independent jurist rather than an ally of the governor.

If that sounds like a stark choice, it is. And Prosser is the one who has made it so.

In announcing his candidacy for a new term, the justice identified himself as a conservative judicial activist who intended to use his position on the court to advance Walker’s agenda. In the first announcement from his re-election effort, the message was clear: “Our campaign efforts will include building an organization that will return Justice Prosser to the bench, protecting the conservative judicial majority and acting as a common sense complement to both the new administration and Legislature.”

Months later, when that line began to stir controversy, Prosser attempted to step back from the statement. But then his campaign said that the April court election is about locking in a “conservative majority” on the court “and nothing more.”

These are unprecedented statements in the history of the Wisconsin judiciary. And they have caused former Prosser backers to distance themselves from the justice, as have recent revelations that suggest the justice has abandoned any pretense of respect for judicial integrity. (Newspapers reported in late March that he had called the chief justice of Wisconsin’s high court, Shirley Abrahamson, a “bitch” while threatening to “destroy” her.)

Forner Governor Pat Lucey, who agreed last year to serve as Prosser’s campaign co-chair, announced Thursday that he had had it with the justice.

“I can no longer in good conscience lend my name and support to Justice Prosser’s candidacy,” explained Lucey. “Too much has come to light that Justice Prosser has lost that most crucial of characteristics for a Supreme Court Justice—as for any judge—even-handed impartiality. Along with that failing has come a disturbing distemper and lack of civility that does not bode well for the High Court in the face of demands that are sure to be placed on it in these times of great political and legal volatility.”

“At the very same time that my confidence in Justice Prosser has waned, I admire and have continued to be impressed with Assistant Attorney General JoAnne Kloppenburg. She has adhered throughout the campaign to even-handedness and non-partisanship and has exhibited both promising judicial temperament and good grace even in the heat of a fierce campaign,” wrote Lucey in a statement broadly circulated in Wisconsin. “For these reasons I have today resigned as Honorary Co-Chairman of Justice Prosser’s campaign, and I endorse Assistant Attorney General JoAnne Kloppenburg for the Wisconsin Supreme Court on April 5, 2011.”

Lucey’s move highlights the seriousness with which Wisconsin’s approach judicial elections, a tradition that places critical choices about how to constitute the courts in the hands of citizens. The point of this tradition, which is deeply rooted in the state’s history and values, is to take politics out of the process. Unlike in other states, where politicians and legal insiders cut backroom deals to pick jurists, Wisconsin’s judicial elections let the people choose justices and then hold them to account.

April 5 will be Prosser’s accountability moment.

And it is his partisanship—as well as his link to a very controversial Republican governor—that has gotten him in political trouble.

A former Assembly speaker, Prosser mentored Walker after the younger man was elected to the Assembly. Wisconsin is not a large state, so relationships of this sort frequently develop.

Most judges resolve conflicts of interest by recusing themselves from specific cases and establishing strict standards. Unfortunately, Prosser has not done this. Instead of abandoning his past role as a partisan legislative leader, he has now positioned himself as a partisan leader on the court.

Consider his involvement in the legal wrangling over Supreme Court Justice Michael Gableman’s violations of the state’s code of conduct for jurists. The Wisconsin Judicial Commission determined that Gableman and his campaign purposely engaged in actions that violated the Code of Judicial Conduct and that “the prosecution of this matter was appropriate and a constitutional application of that valid rule.” However, Prosser blocked that prosecution.

Why? Prosser and Gableman have each benefited from the aid of the out-of-state political interests that backed Walker’s election. Acting as a partisan, Prosser provided Gableman ethical cover in order to tip the balance of the court so as to align it with the Washington-based interests that have guided Walker’s agenda. Prosser’s moves suggest a steady determination to make the high court a judicial rubber stamp for the governor.

For Wisconsinites who prefer the ancient model of governing that put all power in a monarch, Prosser is the right choice. He is running as an explicit supporter of the governor. That is his right. But it is also the right of the voters to set a higher standard.

And they have an opportunity to do so by electing Kloppenburg, who would re-establish the court as an independent branch of government.

Kloppenburg is not a politician. She is an experienced and well-regarded prosecutor; an instructor at the University of Wisconsin Law School; and an advocate with a distinguished record of arguing cases before circuit courts, the Court of Appeals and the Wisconsin Supreme Court.

But her real strength is her nonpartisan record and commitment to judicial integrity.

Kloppenburg offers a reminder that it is still possible to disregard politics and to respect, honor and maintain the rule of law. As a litigator and prosecutor with the Wisconsin Department of Justice since 1989, she has served with two Democratic attorneys general (Jim Doyle and Peg Lautenschlager) and two Republican attorneys general (Don Hanaway and J.B. Van Hollen).

In the course of the current campaign, Kloppenburg has gone out of her way to reject special-interest money and to highlight her commitment to judicial ethics and the code of conduct. She says: “Supreme Court justices should not act as advocates for any cause or group nor as legislators. Rather, Wisconsin residents deserve to have confidence that judges are impartial and independent decision-makers who apply the law fairly and clearly based on the law and the facts. That is what my background and broad legal experience have prepared me to do. That is the kind of justice I will be.”

This is the traditional Wisconsin view, and the standard that voters have respected and defended through most of the state’s history when electing judges.

Now they have an opportunity to defend that standard once more. The April 5 Supreme Court vote provides a stark choice between a partisan judicial activist who would make the court an extension of the governor’s office and a respected legal scholar, litigator and prosecutor who would restore the court’s independence.

JoAnne Kloppenburg has embraced the higher and better standard that Wisconsinites have always demanded of their Supreme Court justices. If she wins her uphill race, in which she has enjoyed strong support from the governor’s critics, it will signal that the hundreds of thousands of citizens who have rallied in the streets are becoming a potent new force in the politics of the state—and, perhaps, of the nation.