Voter suppression efforts by new GOP governors in key swing states are well-known. How much of an impact will they have in the Presidential election? Does Obama start a couple points in the whole in each of those states as a result compared with 2008? Similar question on gerrymandering by GOP legislatures. Have the GOP done a good job of consolidating their gains from 2010 and making it increasingly difficult for Dems to take back the House?I really have no estimate of the size of the effect, at all. My guess would be that it's on the small size -- that a couple of percentage points is too big. But I really don't know.
I suspect the biggest effect isn't voter id or aggressive purging of voter roles or restrictions on voter registration drives; I'd guess it's restrictions on felon and ex-felon voting. Some of which is new, some of which is ongoing. Of course, measuring that one depends on what you compare it to (since lots of people have different views of who should have the franchise), but I think that's the big one. Otherwise, while I agree with those who think it's outrageous to try to win elections by preventing people from voting, I suspect that the overall effects are relatively small.
(Again, anyone lurking who knows the turnout literature more than I do is urged to chime in).
On gerrymandering: What I've seen on this is that it's basically a push. Remember, the last round was after a decent Republican year, so it's not as if we're seeing a brand new shift to Republicans drawing lines in all that many states, and there are some -- California, for one -- that are going the other way.