Bernie Sanders has sent a fundraising plea to his backers for Democrat Russ Feingold, who is running for the Senate in Wisconsin.

The Sanders campaign announced that it had sent the request to his list of supports on Thursday. Feingold has not endorsed Sanders or Hillary Clinton and refused to say who he voted for in Wisconsin's April primary. Feingold has said he will back whoever is the nominee.

Feingold is running against Republican Sen. Ron Johnson in a rematch of their 2010 race.

Sanders in the email calls Feingold a friend who joined him in the Senate in fighting to improve President Barack Obama's health care overhaul and pushing for campaign finance reform. Feingold served 18 years in the Senate before losing to Johnson.