Here’s a look at what could happen.
A resolution of disapproval
Congress could approve a resolution that contests the status of the national emergency Trump has declared. They can do so under the National Emergencies Act of 1976. The resolution, if passed, would stop the plan to divert money from other government programs to build the border wall.
The resolution could pass with a simple majority vote in the House and Senate – 218 votes in the House and 51 in the Senate.
There is a Democrat majority in the House where a resolution could easily pass.
There are 48 Democrat members of the Senate. Democrats would need four Republicans to vote with them to pass a joint resolution.
Reps. Joaquin Castro, D-Texas, and Alexandria Ocasio-Cortez, D-New York, have said they will introduce a bill in the House to block the declaration. By Friday afternoon, Castro told The Washington Post he had gathered more than 60 co-sponsors for the resolution.
Sen. Tammy Duckworth, D-Illinois, told ABC's "This Week" that she believes the Senate has enough votes for such a resolution.
"I think we do," she said. "Now, whether we have enough for an override and veto, that's a different story. But frankly, I think there's enough people in the Senate who are concerned that what he's doing is robbing from the military and the DOD to go build this wall."
If a resolution should pass both chambers of Congress, it would go to the president’s desk for a signature. The president would almost certainly veto the resolution, marking the first time in his term he has used the veto power.
If he does veto the resolution, it would go back to Congress where it would require a two-thirds vote in both the House and the Senate to override the veto. In the House, 290 votes would be needed. In the Senate, the number would be 67.
A lawsuit – or several of them
The president has broad powers under the National Emergencies Act, so until the provisions of Trump's declaration are made public, it's unclear what someone could sue him over concerning the declaration.
But sued he will be -- some suits are already in the works -- and here is where those suits could come from:
Congress: It's likely that House Democrats would sue on grounds that the president overreached his powers by bypassing the power Congress has to control funding for government programs and projects. However, Democrats in Congress would have to first establish that they have the right to sue the White House, and that can be difficult since the president was given the authority to declare a national emergency under the National Emergencies Act in 1976.
The House could challenge Trump's definition of an emergency, but the definition in the National Emergency Act is vague, leaving what is a national emergency pretty much up to the president.
Activist groups: The American Civil Liberties Union said on Friday it plans to sue the president over what they call his "unconstitutional power grab that hurts American communities."
Landowners: Those who own land along the area where the president has proposed a border wall could file suit over the seizure of their property if that happens. However, the government is generally allowed to buy up private property for public use – such as when privately-held land is taken to make room for a freeway. The practice is called eminent domain. It is often an uphill fight for landowners.
States: California Attorney General Xavier Becerra has promised that he will file a suit against the White House claiming that his state will be harmed if Trump diverts funds from other projects to build a wall. He said that four other states, New Mexico, Oregon, Hawaii and Minnesota will join his state in the pending lawsuit.
Nevada's attorney general has also threatened a suit.