His plan has been called radical, ridiculous and "stupid" (that last bit of feedback comes from the Mexican president). One Texas Democrat recently suggested Donald Trump take his idea to build a wall along the U.S.-Mexico border and shove it up his you-know-where.
But the idea of building a wall - or at least some kind of barrier - on the U.S.-Mexico border is hardly new. Over the past few decades, Congress has in fact authorized hundreds of miles of fencing along the border. True, it's not actual brick and mortar. But if Trump gets elected president and Congress stays controlled by Republicans, it's not hard to see how the political will is there for hundreds of miles of fence to turn into Trump's "great, great wall."
I caught up with Alex Nowrasteh, an immigration policy analyst at the Cato Institute, a libertarian think tank, to get a better sense of Congress' approach to border barriers, which has shifted over time from redirecting the flow of illegal immigration to trying to stop it completely. Here's a timeline of the big flashpoints:
1990: The first fence goes up
For decades, U.S. border patrol had been playing an endless game of cat and mouse on the vast, largely empty U.S.-Mexico border: They would catch immigrants crossing illegally, ship them back to the Mexican side of the border, and both sides would repeat.
In 1990, the border patrol shifted its strategy from catching immigrants on the U.S. side to trying to stop them from crossing in the first place. And so the first wall was built.
Actually, it was a fence - a 14-mile-long, about 10-feet-high fence along the San Diego border. There were barriers for vehicles, too, kind of like the guard rails you see along the highway.
"Outside of a port of entry, this was the first physical barrier" along our Mexican border, Nowrasteh said.
1996: Build more fences - but not for the reason you think
In 1996, Congress passed a law that gave the U.S. attorney general and border patrol the authority to build even more barriers along the border.
So up went more fences. But they were mostly congregated around higher population areas, such as San Diego and El Paso. That's because the border patrol agency's immediate concern was to respond to complaints from U.S. residents about immigrants coming onto their private property. Texas' borderland in particular is about 95 percent privately owned.
"Residents were so upset about waking up and finding an illegal immigrant sleeping on their yard," Nowrasteh said, "so to solve public complaints, they built a fence around El Paso and funneled these border-crossers elsewhere."
Put another way, this first series of border walls wasn't aimed at keeping all illegal immigrants out. It was aimed more at directing the flow of traffic away from population centers.
2005 and 2006: Build, baby, build
Illegal immigration from Mexico had been steadily climbing in the '90s, peaking in 2000 at 1.6 million per year, Nowrasteh said. But the 2005-06 period was also a big year. Over a million people were apprehended on the border pretty much every year starting in 2004, in fact.
It's no coincidence that's also when a Republican-controlled Congress, with a Republican in the White House, took its first big step toward fencing off the entire border.
In 2005, they passed a law that waived any legal barriers to putting up fences. If the border patrol wanted to build a barrier that ran into conflict with, say, federal noise control or conservation laws, this new law said the fence took precedence.
The next year, Congress passed a bipartisan law requiring the Department of Homeland Security to build barriers on the border - 850 miles, in fact. Much of it was to be double-fenced, too. In the Senate, 26 Democrats voted for it, including New York's then-junior senator, Hillary Clinton.
At that point, there were only a few dozen miles fenced off, so this new law marked a big shift in how Washington approached securing the border. The message from Congress was clear: Building a physical barrier was an acceptable and even desirable policy solution to illegal immigration.
Public opinion at the time was split, with a 2006 CNN poll showing 45 percent in favor of a 700-mile fence. Support rose to 54 percent in a 2010 CNN poll, though, and Republicans in particular began to demand action.
Even those who had been pragmatic when it comes to immigration reform and giving illegal immigrants a way to stay in the United States felt the pressure to toe the line. "Complete the dang fence," Sen. John McCain, R-Ariz., memorably said in a 2010 campaign ad, as he was facing a more conservative primary challenger.
2013: Lots of border patrol agents, but no fence
Up until this point, we've been talking about actual laws - you know, bills passed by Congress and signed by president. But to talk about today's relationship with the border, we have to reframe the discussion to plain old bills - proposals that have been tried and mostly failed.
That's because Congress has tried and failed for several years now to pass immigration reform. The closest they got was in 2013, when a Democratic-controlled Senate voted to overhaul our immigration system. It didn't call for more fencing, but to appease Republicans who wanted to make sure the border was secure, it planned to send 20,000 more border agents.
Nowrasteh says over the years, talking about a fence or wall has been used interchangeably in American politics. But Trump's talk of a 30, 40, 60-foot wall with a door in it has forced us to make that distinction. And the polls have reflected that. While there has been substantial and even majority support for border fencing, a recent poll showed relatively little support for a massive border wall. Pew asked people earlier this year whether they supported walling off the entire U.S.-Mexico border, and just 34 percent were in favor.
It's also notable that from the end of the 2000s and into today, the flow of illegal immigration has dropped alongside the U.S. housing market and economy. In 2012, for the first time since the Depression, more Mexicans left the United States than entered it. A 2016 nonpartisan Congressional Research Service report said it wasn't clear whether more fences also played a role in the dip in immigration. Some critics of higher border security, such as the Cato Institute, claimed more fences/walls/barriers weren't necessary anymore.
The bill was never brought up in the Republican-controlled House of Representatives anyway.
2016: Trump's not the only one who wants a wall
Fast-forward to the 2016 presidential campaign. Nearly every single Republican presidential candidate talked about how the border needs to be secured before any other immigration laws can change. The senators running for president had long, winding debates about who was more adamant on border security.
As such, a majority of them also wanted a wall built.
The guy who said he'd build the biggest, bestest wall of them all, of course, came out on top. And it's not hard to see how, if he's elected president, this could become a real initiative for his administration - even if it's not 55 feet high and built of bricks.
• 05/26/16 What Bernie Sanders' new endorsements say about his future plans
• 05/11/16 Ted Cruz built a brand on being the most hated man in Washington. Now what?
• 04/20/16 Has Trump finally realized he can't just bulldoze his way to the White House?
• 03/28/16 Do House Dems have a shot at the majority this year?
• 03/14/16 Undersold: Trump would honor Obama's foreign deals (but make them better, of course)
• 02/22/16 The GOP's Supreme Court strategy is dicey. But its potential payoff is massive
• 02/11/16 Who is John Kasich? Five things you should know --- including how to pronounce 'Kasich'
• 12/21/15 Words of caution to temper Ryan's optimism