Civil Rights activists Reverend Jesse Jackson from Chicago and Al Sharpton from New York, both aligned with the Democratic Party, will be in Milwaukee, the state's largest city, to try to spur a strong turnout by black voters.
If he is defeated, Walker would become the third state governor recalled from office during his term, after North Dakota's Lynn Frazier in 1921 and Gray Davis of California in 2003.
Polls show a close race although Walker has held a single-digit lead since the recall date was formally set, and there are almost no undecided voters. The focus is on voter turnout in a state with a history of high voter participation.
During an appearance Sunday on the CNN's "State of the Union" program, Barrett said a tracking poll he'd seen two nights ago showed him just "one vote behind (Walker). Not one percentage point behind but one vote behind."