Clinton was in Milwaukee to support Tom Barrett, the Democratic candidate in Tuesday's recall election, the Chicago Tribune reported. He described Walker as a "dead bang loser" for the state.
Walker angered unions by pushing through legislation severely limiting collective bargaining rights for public employees. Clinton said Walker's policy of cutting taxes for those in the upper brackets is the same one that "doubled the national debt after I left office."
The former president also said a Walker win would encourage the groups outside Wisconsin that have contributed money to his bid to stay in office.
"Everywhere I go in the world, the only thing I see that works is when everyone works together and treats everyone with respect," Clinton said. "That's how you get out of the ditch."
Barrett, the mayor of Milwaukee, is either neck-and-neck with Walker or losing by 7 percentage points, depending on which poll one looks at.
A poll released by the Barrett campaign Thursday indicated Walker led 50 percent to 48 percent, with a margin of error of plus or minus 3.5 percentage points. A poll released by the non-partisan Marquette University Law School Wednesday indicated Barrett was down 52 percent to 45 percent, with a 4.1-point error margin.
Walker beat Barrett 52 percent to 46 percent in the November 2010 race for governor.
Some neutral Wisconsin observers told The Washington Post the polling this time might not be predictive because turnout would likely be unpredictable in a midyear gubernatorial recall election.
Wisconsin election officials have said turnout Tuesday will be high.
Clinton is the highest-profile Democrat to campaign for Barrett.
Obama was to be in Minneapolis and Chicago Friday, mostly for campaign events for his own re-election. Minnesota and Illinois both border Wisconsin.
Walker, sworn in as governor in January 2011, quickly become a national figure with his calls for "repairing" the state budget by shrinking collective-bargaining rights and benefits for public workers.
A union-led revolt led to next week's recall vote.
Walker is much better financed than Barrett, having raised more than $5 million in May and more than $30 million in total, Walker's campaign said.
Barrett raised about $3.1 million in May and $3.9 million since joining the race March 30, his campaign said.
South Carolina Gov. Nikki Haley was to campaign with Walker Friday. She told Fox News Channel last week her state was prospering economically by taking on unions.
"We're going to keep fighting the unions. I'm going to keep being a union buster," Haley told the network. "There's a reason South Carolina is the new 'it' state."
There have been two other gubernatorial recall elections in U.S. history. Republican North Dakota Gov. Lynn Frazier was recalled in 1921 during a dispute about state-owned industries, and Democratic California Gov. Gray Davis was recalled in 2003 over state budget and tax issues.