The argument in Crawford v. Marion County Election Board is about the constitutionality of Indiana's new voter ID law, the most restrictive documentation law in the country.  If it is upheld, then Indiana voters will be required to show a government-issued photo ID with an expiration date or their votes won't count.  That could shut out many Hoosiers; according to a recent University of Washington study, thirteen percent of registered voters in Indiana don't have such IDs.

But that's not all.  The decision in this case will reverberate far beyond Indiana.  For several years now, more than half of the states have seen heated legislative battles over onerous voter ID bills.  A decision to uphold Indiana's law would bolster efforts across the country to enact new ID restrictions.  

If that happens, then millions of eligible voters could be blocked this November.  Studies show that roughly twenty one million Americans don't have government-issued photo IDs.  The voters blocked by photo ID requirements are not evenly spread.  Senior citizens, young people, people with low incomes, and people of color are far less likely than other citizens to have the kinds of IDs required by Indiana's law.  For some groups, the effects would be devastating.  A 2005 study found that 78% of African-American men aged 18-24 in Wisconsin don't have driver's licenses.

No matter what the breakdown, the Supreme Court's decision will affect whose votes will and whose votes won't count.  That could make a big difference in Nov