"If they don't, or if we don't make a deal with Congress, the border's going to be closed, 100 percent," Trump told reporters in the Oval Office, doubling down on a threat he first made at the weekend.
Trump went on to say he was ready to "close large sections of the border, maybe not all of it."
Trump threatened on Friday to close the border with Mexico this week unless Mexico took steps to help the United States with the immigration situation, which had threatened to overload ports of entry in the region.
On Tuesday, Trump spokeswoman Sarah Sanders said, "Eventually, it may be that it's the best decision that we close the border," but that the president "is not working on a specific timeline" and will "look at all of the options on the table."
The Council of Economic Advisers is conducting a number of studies on the potential impact of closing the border and "working with the president to give him those options," she said.
That was a sign of efforts within the administration to slow-walk and minimize the impact of Trump's renewed threat last week to seal the border, and quickly. "I am not kidding around," the president said then, plainly exasperated by the migrant flow.
White House spokesman Hogan Gidley said in an appearance on MSNBC that the White House is now considering a number of options short of a full border closure, including shutting down certain entry ports or parts of all of them. "Everything is on the table," he said.
Even absent the extraordinary step of sealing a national border, delays at border stations have been mounting as some 2,000 border officers assigned to check trucks and cars have been reassigned to deal with migrant crowds, Homeland Security officials said. Average wait times at Brownsville, Texas, were 180 minutes Monday, twice the length of peak times last year.
And when the Otay Mesa, California, entry port closed for the night Monday, 150 trucks were still waiting to get into the U.S.
Arrests along the southern border have skyrocketed in recent months and border agents were on track to make 100,000 arrests or denials of entry in March, a 12-year high. More than half of those are families with children, who require extra care.
Homeland Security Secretary Kirstjen intends to fly to the border mid-week to assess the impact of several changes the administration has recently implemented, including efforts to return more asylum seekers to Mexico as they wait out their case.
On Monday night, Nielsen rushed home from Europe, where she was attending G7 security meetings, to oversee the response to what administration officials are calling a dire emergency at the southern border. Trump is scheduled to make a border trip to Calexico, California, on Friday.
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