A new report on President Trump’s proposed $21.6 billion wall on the border with Mexico is the clearest evidence I have seen so far that Trump’s obsession with undocumented Mexican immigrants is based on false data, and is aimed at stirring up racial panic for political gain.


Until now, most critics of Trump’s planned border wall cited studies showing that about 40 percent of undocumented immigrants have not entered the country by sneaking across the U.S. border with Mexico, but arrived as tourists, and overstayed their visas.


But a new study by the Center for Migration Studies, or CMS, titled “The 2,000 Mile Wall in Search of a Purpose,” shows that the real percentage of visa overstayers is 66 percent, much more than previously thought.


In other words, the vast majority of undocumented immigrants are entering the United States through airports or border checkpoints with valid visas, which would make Trump’s border wall — the centerpiece of his immigration enforcement policy — a monumental waste of money.


“Two-thirds of those who arrived in 2014 did not illegally cross a border, but were admitted after screening on non-immigrant temporary visas, and then overstayed their period of admission,” the report says. “Moreover, this trend in increasing percentages of visa overstays will likely continue into the foreseeable future.”


The report makes it clear that virtually all of Trump’s claims about illegal immigration are based on falsehoods.


Since the start of his presidential campaign, when he said that most of Mexico’s undocumented immigrants are criminals and rapists, the president has been claiming that the country has been invaded by Mexican immigrants, that they are sneaking across an unprotected border and that many of them are “bad hombres” who are raping and killing American children.


Well, it turns out that none of those claims are true. According to the CMS report, there has been “a dramatic decline in the United States undocumented population between 2008 and 2014,” especially from Mexico. Illegal entries from Mexico dropped from 390,000 in the year 2,000 to 110,000 in 2013, it says.


And a growing percentage of those crossing the Mexican border illegally are not Mexicans, but people from El Salvador, Honduras and Guatemala who are fleeing political, drug or gang-related violence. “Many are de facto refugees, not illegal border crossers,” it says.


Which brings us to the crime issue, the pillar of Trump’s immigration scare tactics. Trump routinely cites the cases of mothers and widows of people killed by Mexican immigrants, as he did in his Feb. 27 speech to Congress.


And he has signed an executive order creating an office called VOICE, or Victims of Immigration Crime Engagement, aimed at registering and publishing crimes committed by immigrants, which many critics cited as a Nazi-style effort to falsely blame one particular social group for the country’s crime problems.


But contrary to Trump’s scare campaign, most studies show that immigrants — including Mexicans — are less likely to commit crimes than native Americans.


The Cato Institute, for instance, concluded in a 2015 study that “both the census-driven studies and macro-level studies find that immigrants are less crime-prone than natives with some small potential exceptions.” Among other reasons, undocumented immigrants may commit fewer crimes because of fear of deportation, it says.


So, in view of all of the above, what’s the point of wasting $21.6 billion for a border wall that will have very little impact on reducing illegal immigration?


My opinion: There is no reason for this absurd waste of taxpayers’ money other than Trump’s need to create anti-immigrant hysteria to please his political base, which includes many closet or overt white supremacists.


If Trump wants the number of undocumented immigrants to fall further than it already has, he should put more money into airport screenings and — more importantly — beef up State Department programs to improve the justice system and promote economic development in Central American countries that are the biggest source of illegal immigration. His planned border wall would only be a useless — and very expensive — monument to xenophobic demagoguery.


Andres Oppenheimer is a Latin America correspondent for the Miami Herald, 3511 N.W. 91 Avenue, Doral, Fla. 33172; email: aoppenheimer@miamiherald.com.