It's not unconstitutional to make voters show a photo ID.

The Supreme Court rules, 6-3, in Crawford v. Marion County Election Board:

The voter ID ruling may turn out to be a significant victory for Republicans at election time, since the requirement for proof of identification is likely to fall most heavily on voters long assumed to be identified with the Democrats — particularly, minority and poor voters. The GOP for years has been actively pursuing a campaign against what it calls “voter fraud,” and the Court’s ruling Monday appears to validate that effort....

The plurality opinion leaves room for an "as-applied" challenge, and I'll have more in a little while about what it would take to succeed in such a case.

UPDATE: There are two opinions with 3 votes each and then 2 dissenting opinions. The first opinion is written by Justice Stevens, joined by the Chief Justice and Justice Kennedy. Stevens demands "that burdens on a political party, an individual voter, or a discrete class of voters... however slight... must be justified by relevant and legitimate state interests 'sufficiently weighty to justify the limitation." And the record in this case, which challenged the law on its face, does not show an excessive burden.

What to make of the fact that all the Republicans in the Indiana legislature voted for the law and all the Democrats voted against it?

[I]f a nondiscriminatory law is supported by valid neutral justifications, those justifications should not be disregarded simply because partisan interests may have provided one motivation for the votes of individual legislators.

Since preserving the "integrity and reliability of the electoral process" is a neutral justification for the law, it doesn't matter to constitutional interpretation that Republicans saw partisan advantage in it and Democrats saw the opposite.

The other 3 votes for upholding the law came from Justice Scalia joined by Justices Thomas and Alito. Scalia doesn't approve of the mushiness of Stevens's free-form balancing test, which he thinks invites "endless" litigation:

That sort of detailed judicial supervision of the election process would flout the Constitution’s express commitment of the task to the States.

For Justice Scalia, the "universally applicable requirements of Indiana’s voter-identification law are eminently reasonable," and that is enough. I'm seeing some criticisms of the case that emphasize that Indiana did have enough of a reason to pass such a strict law, but Scalia's point is that the democratic process came up with this law, and there is not enough reason for the courts to overturn it.

I want to look at Justice Breyer's dissent next:

Were I ... to believe, as Justice Stevens believes, that the burden imposed by the Indiana statute on eligible voters who lack photo IDs is indeterminate “on the basis of the record that has been made in this litigation,” or were I to believe, as Justice Scalia believes, that the burden the statute imposes is “minimal” or “justified,” then I too would reject the petitioners’ facial attack... I cannot agree, however...

[A]n Indiana nondriver, most likely to be poor, elderly, or disabled, will find it difficult and expensive to travel to the Bureau of Motor Vehicles, particularly if he or she resides in one of the many Indiana counties lacking a public transportation system. For another, many of these individuals may be uncertain about how to obtain the underlying documentation, usually a passport or a birth certificate, upon which the statute insists. And some may find the costs associated with these documents unduly burdensome (up to $12 for a copy of a birth certificate; up to $100 for a passport). By way of comparison, this Court previously found unconstitutionally burdensome a poll tax of $1.50 (less than $10 today, inflation-adjusted).

Breyer emphasizes that other states with ID requirements are less demanding. Florida, for example, accepts an "employee badge or ID, a debit or credit card, a student ID, a retirement center ID, a neighborhood association ID, and a public assistance ID." Quite simply, Breyer sees an unjustified burden.

Finally, here's Justice Souter's dissenting opinion, which is joined by Justice Ginsburg:

Without a shred of evidence that in-person voter impersonation is a problem in the State, much less a crisis, Indiana has adopted one of the most restrictive photo identification requirements in the country....

[The law] targets the poor and the weak.... [B]eing poor has nothing to do with being qualified to vote.... [T]he onus of the Indiana law is illegitimate just because it correlates with no state interest so well as it does with the object of deterring poorer residents from exercising the franchise.