A number of Republicans were considering write-in bids for the GOP nomination after McCotter's campaign failed to secure enough signatures to get his name on the ballot and the five-term lawmaker decided to retire. A group of local GOP officials unanimously decided, however, to back Cassis.

Cassis has a tough road ahead — it's never easy to run a write-in campaign, and Michigan has fairly strict laws for what counts as a write-in vote. On top of that, Tea Party candidate and reindeer rancher Kerry BentivolioKerry BentivolioIndiana Republican: Leaders duped me Reindeer farmer saves 'cromnibus' with yes vote High drama as .1T spending package advances by one vote MORE (R) is also contesting the race.

If Cassis wins the primary, the district will likely stay in Republican hands — it's one of the more conservative in the state and gave Sen. John McCainJohn Sidney McCainTrump considering pardon for boxing legend after call from Sylvester Stallone GOP poised to advance rules change to speed up Trump nominees The Hill's Morning Report: Inside the Comey memos MORE (R-Ariz.) 50 percent of the vote even after he stopped spending there during his 2008 bid for the presidency. 

Democrats, however, are hopeful they can make the race competitive if Bentivolio is the GOP nominee.