Francis Barry writes for Bloomberg View.

As with those who too easily claim voter id laws affect turnout, this analysis too easily claims it doesn’t.  There are many factors which go into turnout. A competitive Senate race in one state could make a huge difference compared to a state with no race or a non-competitive race [Update: I missed that Barry limited his analysis to states with a competitive Senate or governor’s race.]. States could also differ on a whole host of election rules, such as ease of early or absentee voting (not clear how much either of these affect turnout as well).  But just comparing state to state and the presence or absence of an i.d. law is not a sound way to ferret this out.