If you're wondering the next type of restriction Texas' Republican leadership would like to impose on voters, look at an Arizona case that was argued this week before the U.S. Supreme Court.In a couple of friend-of-the-court briefs, Texas Attorney General Greg Abbott appears to be laying groundwork for requiring proof of citizenship to register to vote.That hurdle isn't required by current state or federal law. And it goes further than the Legislature's mandate to show a photo ID at the polls -- a law whose validity still is being litigated.Unlike in instances where Abbott has led the charge supporting other states whose laws are under court challenge, the AG has only signed onto amicus briefs by Alabama in the case of Arizona v. The InterTribal Council of Arizona, 12-71. (bit.ly/UtancR)Georgia and Kansas were the only other signers on a brief urging the justices to hear the Arizona case; Michigan and Oklahoma joined a later version.The case involves Proposition 200, which Arizona voters approved in 2004 to require proof of citizenship to register to vote. The 9th U.S. Circuit Court of Appeals in April 2012 said Arizona couldn't enforce the provision because it conflicts with the National Voter Registration Act.Congress adopted the NVRA in 2002 to simplify signing up to vote in federal elections. A federal commission developed a standard registration form, and states are limited in the additional kinds of information they can request. (1.usa.gov/cjer4j)The voter application asks for name, address, birth date, phone number and an ID number. In Texas, that can be a driver's license number or last four digits of a Social Security number. For those without either, the state assigns an identifying number.The form also requires applicants to sign and swear they are U.S. citizens and eligible to vote. Lying could result in perjury charges and a fine, imprisonment or deportation.While the integrity of elections is vital in a democracy, it's not clear how Arizona's added requirement improves on what's in place.Alabama, Georgia and Kansas (along with Tennessee) have passed laws similar to Arizona's, so their keen interest in the case is understandable.But Texas? Maybe Texas is simply standing up for state sovereignty against yet another federal intrusion. Abbott makes that argument a lot.Only the politically naive would miss the longer-range motives.