Who Are Michigan Republicans?

Hard to say:

Michigan offers some of the most challenging terrain on which to perform the basic triage of separating likely supporters from likely opponents among those voters who are still up for grabs. It is among a handful of states that does not permit voters to register by party, typically the first attribute campaigns use to categorize individuals. Without any formal indicator of partisan affiliation, targeters look to past participation in primaries for cues—considering those who chose to vote in one party’s nominating contest as either "behavioral Democrats" or "behavioral Republicans." But not enough people participate in primaries for this to be as comprehensive an indicator as party registration is in other states.