Supreme Court justices expressed some skepticism on Monday about an Arizona law that requires people registering to vote in federal elections to show proof of citizenship. 

The legal question before the nine justices is whether the voter registration provision of the 2004 state law is trumped by a federal law, the 1993 National Voter Registration Act, which outlines various ways in which people can register to vote in federal elections. 

That law requires no proof of citizenship. Would-be voters simply sign a statement saying they are citizens.

Based on Monday's oral argument, it was unclear how the court will rule, but a number of justices, including regular swing vote Justice Anthony Kennedy raised some concerns about the law.

Kennedy noted that the basic registration form required under the federal law was intended to make registering to vote relatively straightforward.

He said that the "whole utility of the form is gone" if it is made too complicated by states requiring additional information.

Justice Sonia Sotomayor appeared to agree on that point.

The federal law is intended to "simplify the process," she said. She questioned how requiring additional information could be "consistent with that objective and purpose."

Arizona, which shares a border with Mexico, has a reputation for passing tough anti-immigration laws that have brought it into conflict with the Obama administration.

Various residents, tribal groups and civil rights organizations objected to the law, including the Inter Tribal Council of Arizona.

The case is Arizona v. Inter Tribal Council of Arizona, U.S. Supreme Court, No. 12-71.

This story was originally published on Mon Mar 18, 2013 2:11 PM EDT

Updated at 1:15pm ET A Pennsylvania judge denied a request Wednesday to block enforcement of the state’s new voter identification law.

Judge Robert Simpson said the law “imposes only a limited burden on voters’ rights, and the burden does not outweigh the statute’s plainly legitimate sweep.”

Those seeking to block the law did not show that “disenfranchisement was immediate or inevitable,” wrote Simpson.

In refusing to grant an injunction to stop the law from being enforced, Simpson said he was convinced that state officials were making efforts to inform voters about the law’s requirements and to implement it “in a non-partisan, even-handed manner.”

He also said that based on the availability of absentee voting and provisional ballots as well as court intervention for voters with special hardships, “I am not convinced any of the individual petitioners (seeking to block the law) or other witnesses (at the trial) will not have their votes counted in the general election.”

Simpson conducted a trial in state court in Harrisburg, Pa. after the American Civil Liberties Union and other groups sued to block enforcement of the law which was signed by Republican Gov. Tom Corbett in March.

Referring to one elderly woman seeking to have the law blocked and a student who was a witness at the trial who suffers from autism and other ailments, Simpson wrote, “I would be shocked” if they and other similar voters would not qualify for absentee voting.

Simpson also ruled that granting an injunction to block the law would interfere with the state’s efforts underway this month to educate poll workers and with the state’s TV, mobile phone, mail and billboard campaign to inform voters about the law’s requirements.

Those trying to prevent the law from being enforced – who included the League of Women Voters of Pennsylvania, the National Association for the Advancement of Colored People, and the Homeless Advocacy Project – had a high legal threshold they had to meet: that there was no set of circumstances under which the statute would be valid. Simpson ruled that they failed to meet that test.

He indicated that the plaintiffs could try to show in future litigation – once the law is enforced for the Nov. 6 election and future elections – that it imposes an unconstitutional burden on specific voters.

Nearly 760,000 voters couldn't be matched between Pennsylvania's voter list and the driver's license database

The plaintiffs had cited a remark by Republican House Majority Leader Michael Turzai at a GOP event that the voter ID law "is going to allow Gov. Romney to win the state of Pennsylvania.” But Simpson said that the U.S. Supreme Court, in upholding the Indiana voter ID law in 2008, had said if a law has valid neutral purposes, then “those justifications should not be disregarded simply because partisan interests may have provided one motivation for the votes of individual legislators.”

In his decision to deny the request for an injunction, Simpson relied heavily on the Supreme Court’s decision which upheld the Indiana voter ID law, and on decisions by state supreme courts in Georgia and Michigan upholding their states’ voter ID laws.

The ACLU and other groups said they would file an appeal to the state Supreme Court.

Pollster and political scientist Terry Madonna at Franklin & Marshall College in Lancaster, Pa., who has been a critic of the voter ID law, said that one of the justices on the seven-member Pennsylvania Supreme Court has been placed on administrative leave while being prosecuted for using judicial staff for campaign purposes, so the court now has three Republican members and three Democratic ones.

Simpson’s ruling would stand if the Pennsylvania Supreme Court were to deadlock three-to-three. But Madonna noted that the chief justice of Supreme Court, Ronald Castille, a Republican, did vote last spring with the Democratic justices in an important redistricting decision. Castille “could be the wild card here,” Madonna said.

As for the law’s impact on balloting in November, Madonna said, “I don’t think it will affect the outcome of the election” in Pennsylvania.

Madonna said “there is a monumental debate about how many people are affected by this” voter ID requirement. “No one knows for sure the exact number. Having said that, the election here would have to be very, very close… probably within 50,000 to 75,000” for the law to affect the outcome. “And we don’t have presidential elections in this state decided by 50,000 to 75,000 people,” he said.

The closest presidential election in Pennsylvania in the last 50 years was in 1988 when Republican George H.W. Bush beat Democrat Michael Dukakis by 105,000 votes out of a total vote of 4.5 million. In 2006, Democrat John Kerry carried the state by 144,000 votes out of a total of 5.7 million over George W. Bush.

Election law expert Richard Hasen at the University of California, Irvine, the author of the new book, The Voting Wars, said on his blog that Wednesday’s ruling is “almost certainly going to lead to Pennsylvania’s use of its new voter id law in the November elections.”

He called Simpson’s decision “a careful opinion by a judge who struggled with the evidence and the law and, as I expected, issued a thoughtful, non-ideological and well-done decision.  I disagree with the decision’s bottom line… but there is no doubt this is a judge acting in good faith applying the law and facts as he found them.”

Hasen added that the Department of Justice might try to get a ruling blocking use of the law “raising a violation of section 2 of the Voting Rights Act, (but) I think that such a remedy would be unlikely to be granted at this time.”

An assessment by Pennsylvania Secretary of State Carol Aichele’s office found that 91 percent of the state’s 8.2 million registered voters have Pennsylvania Department of Transportation issued licenses which are acceptable ID for voting.

It also reported that names of nearly 760,000 voters couldn’t be matched between the state’s voter list and the driver’s license database.

Aichele spokesman Nick Winkler said last months that this number included some simple name mismatches between one database and another – “Dave Smith” versus  “David B. Smith” for example – and that some of the 760,000 do in fact possess valid IDs for voting.

The law also says other forms of ID are acceptable, such as military ID cards, U.S. passports, identification cards from accredited Pennsylvania colleges or universities, Pennsylvania senior care facility IDs, or other photo identification cards issued by the federal, Pennsylvania, county or municipal governments.