Kloppenburg, 61, who was elected to the state appeals court in 2012, announced her candidacy for Wisconsin's highest court in an email. Donald formed a campaign committee last week and told The Associated Press in an email that he plans to officially launch his campaign soon.
They are both seeking the seat currently held by Justice Patrick Crooks that is up for election in April.
"We must continue to ensure that our courts are fair, neutral and impartial," Kloppenburg said in the email. "Special interests have spent huge amounts of money trying to influence Wisconsin's judicial system. Those special interests and partisan politics don't belong in our courts. We must ensure that justice isn't for sale. Justice must be non-partisan."
Donald told AP that he is running to bring a fair and independent voice to the Supreme Court, help restore the public's faith in the court and make sure everyone is treated fairly. Donald was first appointed to the court in 1996 by former Republican Gov. Tommy Thompson and re-elected as circuit court judge in April. He previously worked seven years as assistant Milwaukee city attorney.
The 77-year-old Crooks, who declined comment Friday, has not said whether he will seek re-election to another 10-year term. He was first elected to the court in 1996.
While the Supreme Court is officially nonpartisan, it is currently controlled by four conservative justices with two liberals and Crooks, who is seen as a swing vote.
When Kloppenburg ran for the Supreme Court four years ago, it came at the height of the debate over Gov. Scott Walker's law effectively ending collective bargaining for most public workers.
Her election against conservative Justice David Prosser became a referendum on the union law known as Act 10. She lost to Prosser, following a recount, by 6,004 votes.
Candidates for the 2016 race can begin circulating nomination papers in December and they are due on Jan. 5. The election is April 5.
Follow Scott Bauer on Twitter at https://twitter.com/sbauerAP