BOSTON (AP) _ Democrats are looking for a political toehold in their campaign to unseat Republican Gov. Charlie Baker in next year's election.

They think they've found one in the revelation that Baker's education chairman made a previously unacknowledged donation of nearly $500,000 to a group supporting last year's charter school ballot question.

Baker said there's nothing illegal about Paul Sagan's donation, and it's no secret he and Sagan back charter schools.

The donation was disclosed after an investigation by the Office of Campaign and Political Finance forced the group to identify its donors this week.

Newton Mayor Setti Warren, a Democratic candidate for governor, said it's hard to imagine Baker didn't know about what Warren called an ``illegal scheme.''

Democrat Jay Gonzalez, also a gubernatorial candidate, called on Baker ``to come clean.''