President Donald Trump has been promising to build a massive wall along the US-Mexico border since his 2016 campaign, making it the benchmark of his immigration policy.

His push for a border wall ignores one of the biggest issues related to undocumented immigration: roughly half of all people who are in the US illegally came her legally but overstayed their visas.

A 2017 study from the Center for Migration Studies, a nonpartisan think tank, found that since 2007 visa overstays have outnumbered undocumented border crossers by half a million.

President Donald Trump has been promising to build a massive wall along the US-Mexico border since his 2016 campaign, making it the benchmark of his immigration policy.

Trump portrays the wall as a panacea to undocumented immigration and problems he associates it with, including illicit drugs and crime. But the president's push for a border wall ignores one of the biggest factors related to undocumented immigration: roughly half of all people who are in the US illegally are visa overstays.

Read more: Trump threatens to close the border over a new caravan forming in Honduras that reportedly isn't even headed for the US

Simply put, they're people who entered the country legally but their visas have since expired and they've established residency in the US without proper documentation.

In making the case for the border wall, Trump has painted illegal border crossings as the primary source of undocumented immigration, but that is inaccurate.

A 2017 study from the Center for Migration Studies, a nonpartisan think tank, found that since 2007 visa overstays have outnumbered undocumented border crossers by half a million.

The report showed that in 2014 visa overstays accounted for roughly 42% of the total undocumented population (roughly 4.5 million people) and that overstays accounted for about two-thirds (66%) of those who joined the undocumented population that year.

In 2017, over 600,000 travelers who came to the US overstayed their visas and remained in the country by the end of the year, according to the Department of Homeland Security (DHS).

Read more: Border-crossing arrests are at historic lows — but Trump is still bemoaning a 'drastic surge' in illegal immigration

Meanwhile, government data shows the number of people arrested for trying to illegally cross the border in 2017 hit a historic low at just above 300,000. Arrests for illegal border crossings have declined drastically from historic highs.

Annual southern border arrests between 2000 and 2017. Business Insider/Shayanne Gal

The undocumented population in the US in 2016 was around 10.7 million, or 3.3% of all people in the US, representing a 13% decline from the peak of 12.2 million in 2007, according to Pew Research Center.

Accordingly, critics feel the wall would be a waste of money and do little to address the myriad issues surrounding immigration in the US, including visa overstays.

Greg Siskind, an immigration attorney based in Memphis, Tennessee, told INSIDER it's fair to say Trump's border wall is an inadequate, excessively expensive approach to thwarting undocumented immigration.

Alluding to the fact that people have ways of working around a border wall, Siskind added, "And I'm pretty sure that these brand new technologies called 'tunnels' and 'ladders' ... won't stop illegal border crossings either."

Matthew Kolken, an immigration attorney in Buffalo, New York, echoed these sentiments and suggested an alternative to a physical border wall.

"As I like to say the most effective wall is an immigration law that permits immigrants to come here legally," Kolken told INSIDER. "America needs a viable guest worker program that suits the needs of industries with recognized labor shortages, as well as the immigrants who are willing to do the work."

Trump's insistence on obtaining funding to build the border wall has pushed the government into a partial shutdown.

There are conflicting estimates on how much the wall would cost, ranging from $21.6 billion all the way up to $70 billion. Trump has claimed he could build the wall with $15 billion.