UNDATED (WSAU) For the Republican presidential candidates, Sunday was a key day of campaigning before the upcoming Wisconsin primary. But for front-runner Mitt Romney, it was also a stark reminder that it was April Fools’ Day.

Campaign workers took Romney to what he thought was a pancake brunch in the Milwaukee area. House Budget Chairman Paul Ryan went through a curtain to introduce him, while U-S Senator Ron Johnson kept Romney occupied backstage while they waited for Ryan’s cue. His staff warned Romney not to expect a big crowd. And when he and Johnson hit the stage, they saw that the room was completely empty. The real crowd had gathered upstairs, and Romney told them about the April Fools’ prank. He had to, since it was caught on videotape.

Johnson endorsed Romney yesterday, after Ryan did the same on Friday.

Romney and fellow G-O-P candidate Rick Santorum are campaigning for one more day in the Badger State, before the primary tomorrow.