MADISON, Wis. (AP) -- Wisconsin Democrats are shifting tactics again as they work to recall Republican Gov. Scott Walker next month.

They've started playing up questions about whether Milwaukee prosecutors are secretly investigating the Republican governor for campaign or election violations. The probe already has led to charges against a handful of the governor's former aides.

The investigation has been ongoing for months, but it hasn't captivated voters. Democrats hope bringing more attention to it will win them undecided voters. They are expected to be critical when Walker faces Democratic Milwaukee Mayor Tom Barrett in the June 5 election.

Walker has said he's not the target of the probe but he has established a criminal defense fund. He hasn't explained why, but allies have attacked the Democrats' questions as a political stunt.