Opponents of Arizona’s controversial SB1070 law have a new ally: México.

The Mexican government is urging a U.S. court to block a part of the law that prohibits the harboring of undocumented immigrants.

Lawyers representing México asked the 9th Circuit Court of Appeals in a filing Wednesday to uphold a lower-court ruling that blocked police from enforcing the ban. México argued the ban harms diplomatic relations between the United States, undermines the U.S.'s ability to speak to a foreign country with one voice and encourages the marginalization of Mexicans and people who appear to be from Latin America.

"México cannot conduct effective negotiations with the United States when the foreign policy decisions of the federal governments are undermined by the individual policies of individual states," lawyers for the Mexican government said in a friend-of-the-court brief.

The harboring ban was in effect from late July 2010 until U.S. District Judge Susan Bolton blocked its enforcement on Sept. 5. Two weeks before Bolton shelved the ban, she said during a hearing that she knew of no arrests that were made under the provision.

The prohibition has been overshadowed by other parts of the law, including a requirement that went into effect on Sept. 18 that officers, while enforcing other laws, question the immigration status of those suspected of being in the country illegally.

The U.S. Supreme Court upheld the questioning requirement earlier this year, but also struck down other sections of the law, such as a requirement that immigrants obtain or carry immigration registration papers. The nation's highest court didn't consider the harboring ban.

Arizona Gov. Jan Brewer, who signed the measure into law and serves as the statute's chief defender, has asked the appeals court to reverse Bolton's ruling on the harboring ban.

Brewer spokesman Matt Benson said Arizona's harboring ban mirrored federal law and that México was interfering with a matter in U.S. courts.

"México's own immigration laws are significantly more heavy-handed than anything imposed as a result of SB1070. Does the Mexican government believe the nearly identical U.S. federal law harms diplomatic relations between the U.S. and México?" he said.

This wasn't the first time a foreign government has chimed in during disputes over the immigration law.

In 2010, México urged the courts to declare the law unconstitutional, and 10 other Latin American countries had joined in expressing their opposition to the law.

Brewer had said the foreign governments were meddling in an internal legal dispute between the United States and one of its states.

No other countries have joined in México's latest friend-of-court brief.

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