The Republican mayor of El Paso, the largest American city on the US border with Mexico, has warned Donald Trump that if he goes ahead this week with his threat to close the border it would have a “detrimental, almost draconian” impact on the entire region.

“It would be a critical killer to us, frankly”, the mayor, Dee Margo, said.

In an interview with the Guardian at city hall, Margo reflected the views of a community that is now on tenterhooks over the US president’s next move. Trump began threatening last Friday morning that he would shut the border unless the Mexican authorities do more to block Central American asylum seekers from reaching the US.

Margo spoke in impassioned terms about the “trepidation” he was feeling about the threat.

“It would be a crippler. It would be exceptionally detrimental to our economy – I shudder to think about it,” he said.

He added that it was difficult precisely to predict the impact on American families, given that such an event had never happened. But he said: “If you can’t cross the border to get to your job to earn money for your family, that’s a problem.”

Margo’s stark warning carries additional weight in that he is a member of Trump’s own party. The mayor’s grandchildren are fifth-generation El Pasoans.

He said that as a sovereign nation, America needs border security and what he called a “rational” approach to immigration that would include some border fencing, increased personnel in the area, as well as comprehensive immigration reform for those undocumented immigrants already inside the US. Closing the border, he said, was not part of such a rational plan.

From the Pacific coast of southern California to the Gulf of Mexico in eastern Texas, the US-Mexico border is almost 2,000 miles long and runs across scorching desert, desolate mountains, farmland, military ranges and wilderness, along river and gorge, and through towns and cities.

Trump tweeted last Friday: “If Mexico doesn’t immediately stop ALL illegal immigration coming into the United States through our Southern Border, I will be CLOSING … the Border, or large sections of the Border, next week.”

Commuters show their visa cards to US Customs and Border Protection (CBP) agents at the Paso del Norte international bridge to cross into El Paso, Texas, on Monday. Photograph: José Luis González/Reuters

Margo rattled off key statistics showing the interconnected nature of the urban border economies, especially, his own El Paso at the western end of the Texas section of the border and its Mexican twin city directly across the large barrier that divides their downtown areas, Ciudad Juárez. Figures such as:

Annual trade between the two cities amounts to $101bn a year.

23,000 pedestrians cross from Ciudad Juárez to El Paso to work every day.

50,000 residents of El Paso, and 115,000 in the wider region, cross into Mexico to work in the maquiladoras – factories run by US firms that manufacture products then imported by the US.

El Paso has been struggling in recent days with a surge of migrants applying for asylum, arriving from the depressed and crime-ravaged countries of Central America. Customs and Border Protection (CBP) officials prompted an outcry for detaining hundreds of migrants in the open air under the Paso del Norte international bridge that crosses the Rio Grande river into Ciudad Juárez.

Migrants held for days under the bridge described to the Guardian at the weekend having to sleep on the stony ground with only one thin, silvery Mylar emergency-type blanket to share with their children. They were cold and hungry, and many of their babies were sick.

Margo told the Guardian that the treatment of those migrants was “inhumane”. But he said criticism should not be directed at the border protection or Immigration and Customs Enforcement (Ice) federal agents, but against both parties and houses in Congress, as well as Trump.

“The major problem is that there’s been inaction on immigration for 30 years in Washington – they lack intestinal fortitude to do what’s needed, on both sides of the aisles and with presidential leadership.”

He added that the rhetoric from the White House about a possible border closure was exacerbating the current surge in migration from Central America. “My supposition is that there’s been a marketing thrust by the coyotes [people smugglers] in Central America to say: ‘This is your last chance, the border’s going to close, so hop on the bus and let’s go.’”

The result was that thousands of people from Honduras, Guatemala and El Salvador were making “a quick rush to the border. I think the rhetoric coming out of Washington has the ability to control or exacerbate the situation – and right now it’s not good.”

CBP ended on Sunday the practice of detaining people under the bridge and moved the remaining migrants to an undisclosed location. The American Civil Liberties Union issued a formal complaint about children being forced to sleep on the bare ground there, saying conditions were “among the worst we’ve seen in recent history”.

Meanwhile Margo criticized Trump’s insistence that building a wall solves the immigration issue. Even the Texas-Mexico border from El Paso in the west to Brownsville in the east runs close to 1,000 miles.

“A physical structure is part and parcel to border security, but it’s not the panacea. You cannot build a fence from El Paso to Brownsville, Texas – it is a logistical impossibility because the terrain won’t allow it and most of the land along the border is privately owned,” he said.