ANNAPOLIS, Md. (AP) -- Maryland Gov. Martin O'Malley is headed for Wisconsin to rally support for Milwaukee Mayor Tom Barrett in an effort to defeat Gov. Scott Walker in a recall election.

O'Malley, who is chairman of the Democratic Governors Association, told The Associated Press on Thursday as he left the Maryland State House that turnout will be the key to Democratic hopes in the rematch.

O'Malley says the DGA has invested about $3.2 million so far in the race. He says that's more than the DGA invested in 2006 in former Wisconsin Gov. Jim Doyle's re-election and more than in 2010, when Barrett lost to Walker by about 125,000 votes.

The recall election is set for June 5.

O'Malley will follow up the Wisconsin trip with a three-state swing through New England.