By Jeff Mayers

MADISON, Wis (Reuters) - The Wisconsin Supreme Court began hearing arguments on Monday in the legal challenge to the controversial state law that eliminates most collective bargaining rights for public workers.

The measure, which also requires public sector workers to pay more for pensions and health coverage, was passed by the Republican-controlled legislature and signed by Republican Gov Scott Walker in March despite massive public protests.

But it was voided in May by a circuit court judge hearing one of several challenges against it.

That judge, MaryAnn Sumi, agreed with opponents of the measure, who argued the law was invalid because Republican lawmakers had violated Wisconsin's strict open meetings law when they passed the measure using a legislative maneuver.

Arguing before the state's seven-judge Supreme Court on Monday, a lawyer representing the Walker administration argued that an act of the legislature cannot be invalidated by the courts over a violation of the open meetings law because the statute is not a constitutional provision.

Assistant Attorney General Kevin St. John urged the justices to reverse Sumi's ruling because she had overstepped her authority and violated the separation of powers.

Lawyers for both sides were quizzed by the state Supreme Court justices.

One asked St. John why the door was locked for part of the legislative meeting in question.

Another quizzed Sumi's lawyer, Marie Stanton, about the extent of the authority of the courts in enforcing the open meetings law.

One of the seven justices hearing the case was David Prosser, whose recent reelection to the state's High Court had been hotly contested by opponents of the union measure.

In the end, Prosser narrowly defeated his Democratic challenger -- but by less than 0.5 percent of the 1.5 million votes cast and only after a lengthy statewide recount of ballots, the first since 1989.

The union measure propelled Wisconsin to the forefront of a wider national fight as Republicans who took control of many statehouses in last fall's midterm elections moved aggressively to shrink government and made reining in public unions a top priority.

On Friday, the state said six of the Republicans who supported the measure will have to defend their seats in special elections next month after enough signatures were gathered to recall them.

Another three Democratic senators could face recall later this year, too, in what is the largest wave of such special elections in U.S. history.

(Writing by James B. Kelleher)