President Donald Trump declared a national emergency on Friday after Congress declined to fulfill his request for $5.7 billion to help build a wall along the U.S.-Mexico border that was his signature 2016 campaign promise. His move aims to let him spend money appropriated by Congress for other purposes.

On Monday, a coalition of 16 U.S. states led by California sued the Trump administration over the declaration and the president's plan to use billions of dollars to erect a wall. The lawsuit, filed in U.S. District Court for the Northern District of California, came just days after Trump invoked emergency powers Friday.

Trump's National Emergency Declaration: What's Next?

In Washington, D.C., the liberal watchdog group Public Citizen filed a federal suit Friday evening. The lawsuit argued against the constitutionality of the declaration and that Trump exceeded his powers.

Trump's National Emergency Declaration: What's Next?

Democrats, who control the House of Representatives, could either join a lawsuit filed by a third party, or they could file a lawsuit of their own, although the legality of that is in question.

Trump's National Emergency Declaration: What's Next?

Texas Democrat Joaquin Castro, Chairman of the Congressional Hispanic Caucus, plans to introduce a resolution Friday "terminating" Trump's emergency declaration. It would likely pass in the House, but it's future in the Senate is uncertain. Trump threatens a veto if it passes both bodies. Congress would then need a two-thirds majority to override a veto.

reported that at least two more lawsuits may be filed this week. The American Civil Liberties Union has said it will file a case but has not publicly identified its client, according to the report. And Protect Democracy, a watchdog group, and center-right policy institute the Niskanen Center, will file a suit on behalf of El Paso County and the Border Network for Human Rights, the reported.

Any legal challenge is likely to tie up Trump’s efforts in court, delaying the building of a border wall. A protracted legal battle is likely, with an ultimate hearing seen before the U.S. Supreme Court.

In a recent spending bill, Trump asked for $5.7 billion in government funding for a border wall; Congress provided just $1.37 billion. To avoid a government shutdown, Trump signed the legislation, yet declared a national emergency along the border to gain access to other funds. He identified funds, totaling $8 billion.

$1.37 billion: Congress

$3.6 billion: Defense Department, military construction budget

$2.5 billion: Defense Department, drug interdiction efforts

$600 million: Treasury Department, drug forfeiture program