WASHINGTON, Dec 29 (Reuters) - The Biden administration on Wednesday asked the U.S. Supreme Court whether it needed to continue to implement a Trump-era policy that has forced tens of thousands of migrants to wait in Mexico for the resolution of their U.S. asylum cases. Democratic President Joe Biden attempted to scrap his Republican predecessor"s policy - often referred to as "Remain in Mexico" - soon after taking office in January. But after Texas and Missouri sued, a federal judge ruled it had to be reinstated and an appeals court earlier this month agreed. Under the 2019 policy put in place by former President Donald Trump, officially called the Migrant Protection Protocols (MPP), migrants seeking asylum must wait weeks and sometimes years in Mexico for a U.S. court date instead of being allowed to await their hearings in the United States. Biden decried the policy on the campaign trail and immigration advocates have said migrants stuck in dangerous border cities have faced kidnappings and other dangers. After a federal court ruled he had to reinstate MPP, the Biden administration re-issued a memo terminating it in the hopes it would overcome the legal challenges. But the conservative-leaning 5th U.S. Circuit Court of Appeals was not convinced by the new memo, saying in its ruling on Dec. 13, that "simply by typing out a new Word document and posting it on the internet" was not sufficient. Biden"s Justice Department asked the Supreme Court whether it must continue implementing the policy and whether the appeals court erred in concluding the new memo had no legal effect.