U.S. Marines place concertina wire at the Otay Mesa Port of Entry in California on Nov. 11, 2018.

If President Donald Trump follows up on his threat to close the southern border, it could potentially result in millions of dollars in losses to local businesses in regions such as San Diego County. A border closure also could disrupt thousands of truckloads of fresh produce, parts and other products that flow back and forth every day from Mexico to the U.S.

"It would be very damaging to border areas like San Diego," said Lynn Reaser, chief economist for the Fermanian Business and Economic Institute at Point Loma Nazarene University in San Diego. "There is a tight nexus between the southernmost part of California, for example, and Mexico."

During 2017, California exported an average of $127 million per day of merchandise to Mexico, according to the economist. She said the state also imported an average of about $73 million per day of goods from south of the border.

The San Ysidro port crossing between Tijuana, Mexico, and San Diego, where more than 20,000 people and nearly 50,000 cars cross daily, is one of the busiest in the nation. There's also another port of entry about 8 miles east in Otay Mesa, California, where about 3,500 trucks cross daily.

The San Ysidro crossing briefly shut down for a few hours in late November after a caravan of refugees from Central America made their way to the border to request asylum. One estimate released in November by the San Ysidro Chamber of Commerce put the losses from that shutdown at $5.3 million.

Trump tweeted the threat Friday morning, writing "we will be forced to close the Southern Border entirely if the Obstructionist Democrats do not give us the money to finish the Wall & also change the ridiculous immigration laws that our Country is saddled with."

California's Gov.-elect Gavin Newsom responded, tweeting: "$558 billion worth of goods flow across our America's southern border. BILLIONS of dollars — MILLIONS of jobs depend on our border. The President of the United States playing games with our economy and threatening to shut down our border is irresponsible, irrational and absurd."

https://twitter.com/GavinNewsom/status/1078722608583958528

Newsom, a Democrat, is set to take office on Jan. 7.

"San Diego's economy is very much linked with Baja California's economy," said Democratic California Assemblyman Todd Gloria, who represents the San Diego area. "A border closing would be a dagger at the heart of our local economy and many of our job creators and many of our businesses."

Gloria said a study was done a few years ago estimating what cross-border wait times do to the economy and the study showed it was "like not having multiple Super Bowls a year in terms of negative economic impact. Closing it down would just be horrendous in terms of negative economic impact to San Diego."

The closing could have ripple effects along the southern border from El Paso and Laredo in Texas to Otay Mesa and San Ysidro in California. More than one-third of the annual truck crossings take place in Laredo and roughly 15 percent are from the Otay Mesa crossing, according to U.S. Department of Transportation.

U.S. Customs and Border Protection referred questions to the White House for comment. The White House didn't immediately respond to CNBC's request for comment.