As thousands of out-of-state visitors descended on Milwaukee for the start of the Republican National Convention on Monday, many seemed pleasantly surprised by a local delicacy: Spotted Cow beer.

Bartenders on the top floor said Monday’s crowd preferred Wisconsin brewery New Glarus’s Spotted Cow beer—only sold within state lines—but preferences had shifted a bit by Tuesday.

Molson Coors is only selling the beer for a limited time, from March to September. Like New Glarus' Spotted Cow, the beer will be available for purchase only in Wisconsin. “There’s a very ...