MADISON, Wis. -— The effort to recall Wisconsin’s controversial Republican governor is expected to begin Tuesday, although his opponents have yet to come up with a candidate to replace him.

The recall effort comes in response to a Wisconsin law passed earlier this year that effectively ended collective bargaining for most public workers. Gov. Scott Walker’s proposal sparked weeks of protests that drew tens of thousands of people to the state Capitol, and two Republican state senators who supported it were ousted in recalls last summer. Seven other lawmakers targeted for their support or opposition of the law survived recall elections.

Walker, who was elected last fall, isn’t eligible for recall until he has been in office for one year. Democrats have been working closely with union leaders on the effort, and they plan to kick off their petition drive Tuesday. They must gather more than 540,000 signatures by Jan. 17 to force a recall election.

The governor has already started raising money to fight the recall thanks to a donor who filed paperwork Nov. 4 for a fake recall effort. The maneuver allowed Walker to begin accepting unlimited donations.

Meanwhile, potential candidates to replace Walker are jockeying for position behind the scenes and preparing for a primary to narrow the field if the party doesn’t unite behind one person. The possibilities include former U.S. Rep. Dave Obey, Milwaukee Mayor Tom Barrett and state Sen. Jon Erpenbach — one of 14 state senators who fled to Illinois in an ultimately futile effort to block a vote on Walker’s bill. Obey, Barrett and Erpenbach are all Democrats.