It isn't hugely D, but it isn't trending R either. Here's the D% in Michigan compared to their national share of the vote the past few elections.
2000: +2.9%
2004: +3.0%
2008: +4.5%
2012: +3.0%
I'd say the tipping point is >Obama 2012 margin of victory, but <Obama 2008 margin. aka GOP wins something like 52-46.
Whole-number estimates, Michigan has performed since 1992 (the year it officially decamped from a base Republican to a base Democratic presidential state) more bluer than the national margins with the following:
1992: D+2
1996: D+5
2000: D+5
2004: D+6
2008: D+9
2012: D+6Given the Democrats have won the popular vote in all those elections, minus 2004, a Republican is going to have to win nationally by a large national margin (six points may or may not be enough), with base support from the Old Confederacy states, and then see what he/she can get from states that do not normally identify with the party.
If Michigan were to become flipped for such a given Republican-winning presidential election, so too would a host of other states which historically carry the same routinely in presidential elections.
The Republicans have not won the popular vote by a margin greater than George. W. Bush's R+2.46, from 2004, and they haven't once nationally carried the female vote since George Bush in 1988, the very year that Michigan and a host of other states—which haven't carried once for the GOP since after the 1980s—was last in the column for Team Red.
In its voting history, dating back to 1836, Michigan has participated in 45 presidential elections. It carried for the winner in 34 of them. It carried for popular-vote winner Al Gore in 2000. (And, yes, I would give half-credit to states which carried for popular-vote winners because normally a prevailing presidential candidate clears the threshold there and with the Electoral College.) That is right around 75 percent. And there are states with even better records—ones with Michigan routinely carries the same in presidential elections. An example is Pennsylvania, which actually rates in the 80s and is a remarkable companion state. (They've disagreed only in five cycles. Three of them involved a major-party nominee whose home state was one of these two who did not carry the other state. The other two occurrences were during the Franklin Roosevelt era when both states carried three times but colored red not timed for the cycle.)
Michigan is a state that has carried for the winner on average three of every four election cycles. And it is an example, in a realigning presidential period now favoring the Democrats, where having your base in the Old Confederacy (by comparison, Texas's record is historically in the 50-percentile range) is at the disadvantage. The Republican Party of Lincoln, Teddy, and Ike had their base in states like Michigan, Pennsylvania, and Vermont. Now the Republican Party has its base in states like Texas, Mississippi, and Alabama.
The Republican Party will not have Michigan become a base state for the party, once again, unless the Old Confederacy realigns back to the Democratic Party. Realignments inside both parties would have to happen for that to become feasible.