US President Donald Trump visited the southern border with Mexico on Thursday in an effort to boost the case for a proposed border wall.
Trump held a roundtable at McAllen, Texas, in the Rio Grande Valley, the busiest section of the frontier for illegal crossings.
Claiming to be under attack from "the fake news media", the president cited history to justify the effectiveness of his milestone policy.
"They say a wall is medieval - well so is a wheel, a wheel is older than a wall," he argued.
"I looked at every single car out there, even the really expensive ones that the Secret Service users and believe me they are expensive, I said, 'Do they all have wheels?' Yes.
Trump has threatened threatened to bypass congress and declare a national emergency if he and the Democrats fail to reach an agreement on funding for a border wall.
Negotiations with the opposition collapsed on Tuesday, extending the federal stalemate which has shut down the government for 20 days.