According to a study done by the Brennan Center For Justice, as many as 5 million voters will be disenfranchised by Voter ID laws passed in Republican states.
I’ve discussed this before (last paragraph), but the guys over at The Monkey Cage have a post about this new study, as well as the previous studies (here). Summary: there have been several studies done on the effect of voter ID laws on turnout, and there is nothing conclusive which can be drawn from them. Some show a negative effect, some a positive, and some none at all. It’s very difficult to prove what the effect is. Long story short: we have no idea what the impact will be right now.