Washington (CNN) White House lawyers have begun laying the groundwork for the legal defense of a national emergency declaration on the southern border, officials familiar with the matter said, acknowledging the option remains a likelihood should President Donald Trump and Democrats not reach an agreement that would reopen the government.

Such a declaration would be subject to immediate legal challenge -- a reality the President's legal aides have anticipated and already sought to build a defense against, according to the officials.

That has included advising the President's aides on ramping up talk of a humanitarian and security "crisis" at the border, a characterization administration lawyers could use later in court to defend a national emergency.

Trump and others in the White House began using that term more frequently over the past week. The lawyers have suggested the more times the term is used, the more citations they will have in filing a legal defense.

Trump's critics have said any talk of a "security crisis" is manufactured, citing statistics showing border crossings have decreased over the past decade. And they have pointed to administration numbers that indicate a terror threat on the border is overblown.

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