A day before going to the U.S. border with Mexico, President Donald Trump again publicly warned the Mexican government that unless officials crack down on illegal immigrants and illegal drugs destined for the United States, Mr. Trump said he would slap tariffs on cars built in Mexico, and ultimately close the border to trade if Mexican efforts fall short.

"We're going to give them a one year warning," the President told reporters at the White House, seemingly backing away from his recent threat to shut down the border in coming days or weeks.

"If Mexico doesn't give the help - that's okay - we're going to tariff their cars coming into the United States," Mr. Trump said.

"The cars are very big - and if that doesn't work, we're going to close the border," the President said. "But I think that will work - it's massive numbers of dollars."

"I'll do it," the President said bluntly. "I don't play games."

On Capitol Hill, members of both parties expressed concern about the economic impact of any move to close the border.

“Closing the border to legal commerce would be devastating to Texas,” said Sen. Ted Cruz (R-TX).  “Millions of jobs, in Texas and across the country, depend upon trade with Mexico, and the federal government shouldn’t do anything to jeopardize those jobs.”

“We sell lots of pork into Mexico,” said Rep. Frank Lucas (R-OK).  “The trade relationship between the southwestern states and Mexico is critically important.”

In his remarks on Thursday, President Trump seemingly backed off an immediate call to close the border - as he made clear a 'one year warning' was on the table for the Mexican government.

“And if the drugs don't stop - or largely stop - we're going to put tariffs on Mexico and products, in particular cars,” Mr. Trump said.

“The whole ballgame is cars,” he added.