Arguably the most successful element of Donald Trump's presidency involves a campaign promise that hasn't been enacted or funded by Congress. The mere threat of a wall — a "big, beautiful" wall — along the Mexican border has transformed the immigration equation, as fewer undocumented immigrants cross the border into the United States.
The number of illegal border crossings was down 47 percent in July compared with the same period last year, and the number is down 22 percent for the 2017 fiscal year, according to the Department of Homeland Security. Meanwhile, the number of total orders for voluntary departures or removal of undocumented immigrants between Feb. 1 and July 31 is up 31 percent.
In other words, the policy is working even if no bill has made it to the Resolute Desk in the Oval Office desk favored by many presidents.
At Thursday's White House press briefing, reporters questioned press secretary Sarah Sanders about the wisdom of Trump threatening to shut down the federal government if Congress does not fund his plans for the wall. After all, Trump promised Mexico would pay for it.
Reporters also frequently ask whether building a wall is a smart use of taxpayer dollars given that illegal immigration crossings are down. Ditto for Trump's plan to hire 10,000 new Immigration and Customs Enforcement officers as well as 5,000 Border Patrol agents.
Those are questions even people who want to boost enforcement against illegal immigration have been asking.
The spurt in unauthorized immigration reached a high of 12.2 million in 2007, according to the Pew Research Center. During President Barack Obama's tenure, the number sagged from 11.3 million in 2009 to 11 million in 2015.
That is, the number of unauthorized immigrants fell even as the president argued for laws that set a path to legalization for undocumented immigrants and signed executive orders that authorized undocumented immigrants to live and work in the United States.
Trump did not roll out the welcome mat when he announced his candidacy in 2015. He said Mexico was not sending its best residents to the United States, but rather, drug dealers, criminals and rapists. Trump promised a wall and tough immigration enforcement.
After winning election, Trump wavered on his campaign pledge to revoke Obama's executive order that shielded some individuals brought into the U.S. as the minor children of undocumented immigrants. But Trump never stopped pushing for his "big, beautiful" wall or his plans to boost enforcement.
It's no wonder then that the Border Patrol has apprehended markedly fewer individuals trying to cross the border illegally. By focusing on enforcement beyond the border, the Trump plan reduces the incentive for immigrants to come to the United States without authorization.
"Individuals who once thought that if they could get by the Border Patrol or get smuggled in and not apprehended they were home free, that is no longer the case," an administration official said. "An individual who crosses the border illegally and gets past the Border Patrol should no longer feel secure that they're immune from any future enforcement."
So does that mean Trump should abandon his pricey plan to hire more staff for ICE and the Border Patrol?
Don't expect knee-jerk support for the Trump hires from Mark Krikorian, executive director of the pro-enforcement Center for Immigration Studies. When governments decide to hire too many cops too quickly, they sometimes drop their standards, Krikorian warned.
What Krikorian calls "the Trump effect" could argue for the hiring of fewer extra Border Patrol agents, he said, but perhaps more ICE officers responsible for enforcing immigration laws beyond the border. Krikorian also wonders whether the Trump effect might wear off, which could result in a surge of illegal border crossings.
An administration official who spoke on background during a Tuesday press call argued that immigration enforcement has been historically understaffed for its oversize job, so the new positions should be filled. Krikorian also sees the value in using new employees to bury a backlog of nearly 1 million cases that developed during years of chronic understaffing.
And the wall itself? When Trump visited Yuma, Arizona, last week, officials noted that authorities expanded the border fence in 2006 from 5.2 miles to 63 miles and border apprehensions fell by 83 percent.
Krikorian never was a big enthusiast of the fence promise.
"I'm not averse in principle to additional barriers along the border," Krikorian said. "I just don't necessarily think it's job one."
"There are places where we have a decent fence but we need a second layer. There are places where we have just a joke fence and it's falling apart," Krikorian added. And there are places where nature takes away the need for a fence or other barrier.
Of course, Krikorian warned, the Trump effect can discourage illegal immigration only as long as the rest of the world believes Trump will ride enforcement hard. Words alone cannot do the trick.