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 on Nov. 4 for a fake recall effort. The maneuver allowed Walker to begin accepting unlimited donations.

Nicole Larson, spokeswoman for the state Republican Party, responded to the recall effort by saying Walker "remains completely focused on the task at hand _ saving taxpayer dollars and creating a business friendly climate so Wisconsinites can get back to work."

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. Former Dane County Executive Kathleen Falk also could be a contender. Her home area includes Madison, the state capital.

Democratic strategists would love to convince one of the two biggest names in Democratic politics _ former U.S. Sen. Russ Feingold and retiring U.S. Sen. Herb Kohl _ to run, but they believe they could win with a candidate who has some name recognition even if the person has never held statewide office.

"I think people really do, at some level, believe a rocking chair with nobody sitting in it would be a better governor than Scott Walker," Democratic Party strategist Sachin Chheda said.

Marty Beil, executive director of the 23,000-member Wisconsin State Employees Union, said he's not worried about Democrats not yet coalescing around a candidate.

"I believe that we get the signatures, then we get the candidate," Beil said.

But former Democratic Party chairman Joe Wineke said that although he didn't feel a rush to choose a nominee, some party members are "very nervous about the lack of a defined candidate" as petitions are being circulated.

Neither Feingold nor Kohl appears interested. Feingold, who lost his bid for reelection last year, said he wouldn't run for anything in 2012, and Kohl's spokesman has said the 76-year-old senator has no desire to run. He's retiring when his Senate term ends next year.

Obey said he would like to see Kohl or Barrett run to replace Walker, but if they don't, he won't rule out running himself. The 73-year-old served more than 40 years in Congress and was a powerful force behind the scenes in Democratic politics for decades.

"We'll cross if we come to it," Obey said. "Right now, my main purpose is to try to convince one of them to run and see to it that people remember there's a huge amount at stake and we cannot afford to have the opposition to the governor split in different directions."

To that end, he said the priority must be making it clear to the public that the recall effort is moving forward.

"The issue is Scott Walker," Obey said. "The issue is not candidate A or candidate B."