Because there is the likelihood (IANAL) that the discussion and planning of their illegal actions makes what they did a conspiracy. And that seems to be a felony under Wisconsin law. So what happens to a Wisconsin state legislator convicted of a felony?

As you'd be correct in assuming, they are removed from their legislative duties and privileges without any other action required. So if this is true, 18 of 19 Republican Senators and any Assemblyman present who participated in the planning and either did not speak against the action or who voted for it can be charged with a felony. Any of them found guilty would be immediately a private citizen facing sentencing and prevented from ever holding a public office again. Maybe not even Chairperson of their public school district's PTA.

O frabjous day! Callooh! Callay!'

He chortled in his joy.