WEST PALM BEACH, Fla. (AP) — President Donald Trump is prepared to issue the first veto of his term if Congress votes to disapprove his declaration of a national emergency along the U.S.-Mexico border, a top White House adviser said on Sunday.

White House senior adviser Stephen Miller told "Fox News Sunday" that "the president is going to protect his national emergency declaration." Asked if that meant Trump was ready to veto a resolution of disapproval, Miller added, "He's going to protect his national emergency declaration, guaranteed."

The West Wing is digging in for fights on multiple fronts as the president's effort to go around Congress to fund his long-promised border wall faces bipartisan criticism and multiple legal challenges. After lawmakers in both parties blocked his requests for billions of dollars to fulfill his signature campaign pledge, Trump's declared national emergency Friday shifts billions of federal dollars earmarked for military construction to the border.

California Attorney General Xavier Becerra told ABC's "This Week" that his state would sue "imminently" to block the order, after the American Civil Liberties Union and the nonprofit watchdog group Public Citizen announced Friday they were taking legal action.

Democrats are planning to introduce a resolution disapproving of the declaration once Congress returns to session and it is likely to pass both chambers. Several Republican senators are already indicating they would vote against Trump — though there do not yet appear to be enough votes to override a veto by the president.