Former MI Supreme Court Justice Bob Young has dropped out of the race citing financial concerns. This means the Republicans will nominate yet another unknown Republican candidate who Stabenow will likely run up 60% on. As I've said before, it amazes me that the Republicans who control everything else in this state haven't been able to find a credible candidate since 1994. They have quite the bench for governor, but they can't get a real candidate for Senate.
Abraham went down in 2000 pretty much because it was a good year for Senate Democrats. Since then, they've nominated two unknown right-wing bozos who both got crushed.
They probably read the tea leaves and know governor is the better bet. In the alternate universe where Hillary wins Michigan by <1% and the presidency, my guess is a strong challenger would've jumped in.