Private interest groups have spent record levels on an upcoming election for a seat on the Wisconsin Supreme Court, according to the Brennan Center for Justice at New York University School of Law.

The April 1 election pits Wisconsin Supreme Court Justice Louis Butler against Burnett County Circuit Court Judge Michael Gableman.

From Feb. 20 to March 16, Wisconsin voters saw 4,789 campaign ads that cost about $1.58 million, most of them paid for by private interest groups.

Combined, both candidates have spent about $86,938 on advertising during that same period.

The Greater Wisconsin Committee spent $408,293 for Butler, while the Wisconsin Club for Growth spent $384,732 for Gableman, according to the Brennan Center.

Some ads have turned ugly. A complaint recently was filed over an ad that criticizes Butler for his work on a criminal case while failing to mention that Butler had been serving as a publicly appointed defense counsel. Butler has paid for ads criticizing Gableman’s “despicable attacks” on his campaign. Another ad, paid for by the Greater Wisconsin Committee, criticizes Gableman’s handling of several child sexual assault cases, according to the Brennan Center.

Business groups in Wisconsin have criticized the state’s highest court for issuing rulings that overturn caps on medical malpractice awards, ease restrictions on punitive damages and expand the “risk contribution” theory to include lead paint manufacturers.