Retiring U.S. Sen. Herb Kohl is supporting Milwaukee Mayor Tom Barrett in the Democratic recall election primary.

Many Democrats wanted Kohl to run against Republican Gov. Scott Walker, but he stayed on the sidelines.

Kohl said in a statement Wednesday that he doesn't normally endorse in a primary, but he is this time because this is no ordinary election.

The endorsement comes on the heels of the AFL-CIO announcing it was backing Barrett's Democratic challenger Kathleen Falk. The AFL-CIO has 250,000 members through its affiliate unions in Wisconsin. Every union that has endorsed so far has backed Falk.

Meanwhile, Barrett has released his first television ad in the race. In his ad that began running Tuesday night, Barrett doesn't mention his Democratic challengers but lashes out at Walker, saying he created a civil war in Wisconsin.

Barrett, Falk and two other Democrats in the race -- Secretary of State Doug La Follette and state Sen. Kathleen Vinehout -- were scheduled to appear together at a Wednesday night forum.

The campaign is on hyper drive with the May 8 Democratic primary is less than four weeks away. The general election, where Walker will face whoever emerges from the Democratic primary, is June 5.