2. Why not? I don't see how it could damage him. No Democrat is going to be upset
by a visit from the President, and no Walker Supporter is going to vote for him anyhow. But people on the fence, Independents who lean left eg, might be convinced to go out and vote if they see the President there, making it clear how important this election is.
In fact, not going might be more harmful. Democrats in Wisc have complained about the lack of support from the Dem Party. These people have been on the front lines fighting off the Kock Brothers, and not just for their state. It is a super important election for Democrats. A win for Walker would be devastating.