Reported in U.S. Rep. Glenn Grothman is back home, admiring Israeli optimism. (Wisconsin Jewish Chronicle, 8/31/2017)
First of all, Grothman had 2 opponents in 2016: Democrat Sarah Lloyd and Independent Jeff Dahlke. He won quite a bit less than 99% of the vote.
Source: Ballotpedia
And neither did Grothman run unopposed in 2014, when he was first elected to Congress.
It should be noted by Scott Olmer announced as a Democrat but switched parties in mid-August.