WASHINGTON — It would take ‘‘something dramatic’’ in the coming days to persuade President Trump not to close the US-Mexico border, acting White House chief of staff Mick Mulvaney said Sunday, and White House counselor Kellyanne Conway said the president’s threat ‘‘certainly isn’t a bluff.’’

The two senior staffers, appearing separately on Sunday morning talk shows, also reiterated the administration’s intention to cut off hundreds of millions of dollars in assistance — including programs designed to curb gang violence — to the ‘‘Northern Triangle’’ countries of Guatemala, Honduras, and El Salvador.

Those countries are the primary source of tens of thousands of migrants, including caravans of families with children, who have been presenting themselves at ports of entry and asking for political asylum in an escalating humanitarian crisis at the border.

‘‘Democrats didn’t believe us a month ago, two months ago when we said what was happening at the border was a crisis, a humanitarian crisis, a security crisis,’’ Mulvaney said on ABC News’ ‘‘This Week.’’ He said the administration is talking about closing the border because ‘‘we need the people from the ports of entry to go out and patrol in the desert, where we don’t have any wall.’’

He also called on the Mexican government to solidify its southern border and said Guatemala, Honduras, and El Salvador need to do more to prevent their citizens from entering Mexico. If they cannot do that, he said, ‘‘it makes very little sense for us to continue to send them aid.’’

Conway, appearing on ‘‘Fox News Sunday,’’ pushed back against the notion that cutting aid to those countries would make matters worse. ‘‘The conditions are already awful,’’ she said. ‘‘The executive branch has done so much to try to mitigate these awful circumstances, and we need to send a message back to these countries, too.’’

Closing the border is a drastic measure that would have immediate consequences not only for families seeking asylum but also for trade and commerce between the United States and Mexico. Mexico is the third-largest trading partner of the United States, with more than $611 billion in cross-border trade last year, according to the Commerce Department. At the port of Calexico East, in California, more than 1,000 trucks cross the border each day. Laredo, Texas, sees more than 11 trains each day cross the border, according to the US Transportation Department.

If the border closure applied to goods and vehicles as well as people, the economic consequences would be immediate and severe, with automakers and American farmers among the first to feel the pain, according to trade specialists.

‘‘It’s unworkable and unrealistic, and I don’t think he could really do it,’’ said Rufus Yerxa, president of the National Foreign Trade Council, which represents multinational corporations, on Sunday.

Closing the flow of people and goods between the United States and Mexico would interrupt the flow of parts headed to American factories, which could bring some production to a halt. Likewise, refrigerated trucks full of beef and other perishable commodities would jam border crossings.

‘‘The first question would be: Where do you put it?’’ said William Reinsch, a former Commerce Department official. ‘‘Stuff is going to stack up at the border because it’s already on the way there.’’

To deal with ‘‘an unprecedented humanitarian and border security crisis all along our Southwest border,’’ the agency said it had redeployed 750 border agents.

Senator Richard Durbin, an Illinois Democrat, appearing on NBC News’ ‘‘Meet the Press,’’ said, ‘‘When the president says he’s going to close the border, that is a totally unrealistic boast on his part. What we need to do is focus on what’s happening in Central America.’’

Durbin said the government needs to prioritize the humanitarian crisis unfolding along the US-Mexico border: ‘‘The first thing we need to do is meet the humanitarian needs at the border instead of building fences two or three years in the future by taking money from the Department of Defense, focus on facilities to serve these families so that there aren’t children who are hurt and dying as a result of this situation.’’

Senator Bernie Sanders, a Vermont independent and a 2020 presidential candidate, said on CBS News’ ‘‘Face the Nation’’ that there is a ‘‘terrible humanitarian crisis’’ at the border and that the United States needs comprehensive immigration reform. He added, ‘‘We need to make sure that our borders are secure, but also we need a humane policy at the border in which we are not yanking tiny children from the arms of their mothers.’’

Trump took to Twitter on Saturday to implore Mexico to ‘‘stop the many thousands of people trying to get into the USA.’’ He wrote: ‘‘Our detention areas are maxed out & we will take no more illegals. Next step is to close the Border! This will also help us with stopping the Drug flow from Mexico!’’