I agree with Orin that the Supreme Court's Voter ID decision shares a structural similarity with that in McConnell v. FEC. It seems to me that another parallel between the two cases is the extent to which voter perceptions about the integrity (or lack thereof) can form the basis of a governmental interest that can justify regulation of election-related activities.
In McConnell, the Court held that the government had an important interest in preventing "the eroding of public confidence in the electoral process through the appearance of corruption," that could justify regulation of campaign contributions and campaign-related speech. Similarly in Crawford, the Court held that public concerns about election integrity were a legitimate government interest that could justify a voter identification requirement. In each case, this interest is independent of any actual threat to campaign or election integrity, and arguments that the measures in question are unwise or ineffective at preventing actual threats are not particularly responsive. What matters is that the measures in question have the potential to increase public confidence in the electoral system as a whole.
In the voter identification context, this would suggest that it does not matter whether absentee ballot fraud poses a far greater threat to election integrity than in-person voter fraud. Nor does it matter if a voter identification requirement will not deter enough voter fraud to alter election outcomes. What matters is that the average American voter believes that such a requirement is a common-sense measure to prevent fraudulent voting, and that if an ID is required for everyday economic transactions, it can surely be required when voters participate in collective decisions about how our government is to be run.
I am more sympathetic to the Court's decision in Crawford than in McConnell, but I think this parallel between the two cases is quite interesting, and may have important implications for the constitutionality of additional election reforms going forward.