“Since Michelle was there simply as a voter...”
You are incorrect. No one is allowed to campaign within a certain number of feet from the poll, and certainly not while inside the poll location.
However, like others have pointed out, she is the First Lady and she will be given a pass on this.
I know the same law with virtually the same language in Virginia means I, as a private citizen, can wait in line at the polls and talk stuff with my friends also waiting inline.
A party official can't.