From the city of Milwaukee's Voting Information Guide:
Any person registering to vote within the 20 days preceding an election (or after the close of registration) must provide an acceptable proof of residence document as listed below.So that set me to thinking. When was the University of Wisconsin's graduation?
4. REGISTER AT YOUR VOTING SITE ON ELECTION DAY
You may register and vote at your voting site on any Election Day. To determine your voting site, click http://itmdapps.ci.mil.wi.us/electedreps/electrep.jsp and enter your voter registration address.
Any person registering to vote and voting on Election Day must have lived at their current address for at least 28 days before the election with no present intent to move and must provide an acceptable proof of residence document from the list below.
Answer: May 18, 19, and 20. Which is to say, less than 28 days ago.
Which is to say, if you're one of the (heavily left-leaning) many thousands of people who graduated from the University of Wisconsin and then moved (like, out of a dorm, back with your parents, anything), you weren't allowed to register to vote on election day.
Gotta say: that kind of sucks. Unsurprising, but it still sucks.