The U.S. has a free-trade agreement with Mexico, so Trump must declare a national emergency at the border to enact the tariffs. Congress can override the declaration by passing a resolution that Trump could veto; a two-thirds vote of both houses would be needed to override the veto, and that would be the biggest rebuke ever by this Republican Congress to this Republican president.
"Congress passed such a resolution in March after Trump reallocated the border wall funds, but he vetoed it. Now, as frustration on Capitol Hill grows over Trump’s latest tariff threat, a second vote could potentially command a veto-proof majority to nullify the national emergency, which in turn could undercut both the border-wall effort and the new tariffs," the Post reports. "The vote, which would be the GOP’s most dramatic act of defiance since Trump took office, could also have the effect of blocking billions of dollars in border-wall funding that the president had announced in February when he declared a national emergency at the southern border."