For either a ballot or a firearm, requiring photo ID makes it more difficult to obtain. To me, under the practical constraints that currently exist, this is unacceptable for the former but not for the latter. There are a number of reasons:
1) Voting is more fundamental to our democracy than is shooting. Voting has been recognized as a nearly-universal right (excepting felons, genuine incompetents, etc.) for much of our history, whereas owning a particular firearm has not. Even the recent Supreme Court decision only (and by the narrowest of margins) struck down very far-reaching and expansive handgun bans, while permitting states to deny handguns of particular types or to particular people. Further, voting serves as a very effective (comparatively, if not absolutely) way of holding elected officials accountable to the popular will, whereas gun ownership simply does not (despite calls for "Second Amendment remedies").
2) The purpose of requiring an ID is so that the most dangerous individuals will be denied access to either the ballot or the firearm. A ballot in the hand of a dangerous voter is simply not as dangerous as a firearm in the hand of a dangerous shooter. Some people in this thread have disputed this, so let's examine it:
What constitutes the most dangerous voter? Presumably, someone who deliberately tries to select the worse of the two presidential candidates. Estimates indicate that the odds of such a person affecting the outcome of the election are one in the tens of millions (
http://www.nber.org/papers/w15220.pdf).
So what is the danger of selecting the weaker of two candidates in a close election (in a non-close election, a single voter won't matter anyway)? It must be quite low because, knowing nothing else other than the candidates' popularity, we must assume them to be very similar. Even if they aren't, I don't see how, for example, the expected difference between a President Obama and a President Romney could account for more than a few thousand lives, and that in a murky and attenuated way. Thus, a malicious voter could effectively "kill" (again, in an attenuated way) about .001 people. Of course, few ineligible voters are actively malicious (and some eligible voters *are*, for that matter), so the real benefit of denying ineligible voters is quite a bit smaller.
In contrast, we have fairly good way of identifying dangerous shooters, particularly those who have histories of killing multiple people with guns. The most dangerous shooter can easily be expected to kill multiple people with guns, thousands of times more than the most dangerous voter.
So, by my estimate on the scale dangerous things, ballots fall not only under firearms but under knives and blunt objects. This motivates the question, for those who feel that ballots are too dangerous for people without easy access to government IDs:
"Why do you want to require photo ID to obtain ballots, but not to obtain knives?"