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SAN FRANCISCO (Reuters) - California filed a lawsuit on Wednesday over the Trump administration’s plan to construct a wall along the border with Mexico, adding to the obstacles facing one of the Republican president’s key campaign promises.

President Donald Trump has insisted Mexico would pay for building the wall, which experts said could cost about $22 billion and take more than three years to complete.

With Mexico refusing to pay, Trump has said since taking office in January that the wall will initially need U.S. funding but that he will find a way to make Mexico ultimately pay for it.

Democrats in the U.S. Congress, however, firmly oppose the border wall, and at least some Democratic senators would need to vote for its inclusion in a spending package.

Democratic attorneys general including California’s Xavier Becerra have sued the Trump administration on a range of issues.

The border wall lawsuit filed in a San Diego federal court alleges that Trump’s wall violates federal environmental standards, as well as constitutional provisions regarding the separation of powers and states’ rights.

Representatives for the U.S. Justice Department declined to immediately comment on the case.

The lawsuit asks a judge to stop the administration from building the wall until it demonstrates compliance with environmental laws, and issue an order that the Department of Homeland Security cannot waive any federal guidelines in order to facilitate the project.

Last month the Trump administration said it had selected four construction companies to build concrete prototypes for a wall, which will be will be 30 feet (9 meters) tall and about 30 feet wide and will be tested in San Diego.