The US Court of Appeals for the Ninth Circuit held late this week that Arizona may enforce its voter identification law while a non-profit organization challenges the law in federal court. The law, which Arizonans approved in 2004 as Proposition 200, requires voters to show a government-issued ID at the polls. The Ninth Circuit ruling was filed Thursday. Opponents of the law have called it a "21st century poll tax," since it requires people to purchase photo ID cards, and have argued that the law places an unconstitutional burden on minority, immigrant and elderly voters. Proponents say it prevents illegal immigrants from casting ballots. Last year, the US Supreme Court ruled that Arizona could enforce the law at the polls for the November elections, reversing a Ninth Circuit decision rendered earlier that month.