At construction sites across Houston, atop the skeletons of soon-to-be glistening towers, in ditches cut through roadways, amid the frames of rising townhomes and across the burgeoning suburbs, tens of thousands of laborers build or rebuild the sprawling metro region.

Many of them are immigrants in the country illegally.

In Texas, an estimated 400,000 construction workers reside illegally, according to one study. If they were forced to leave the country, contractors say, state construction companies would face a difficult fallout, including higher labor costs, construction delays, and some projects canceled altogether.

"Texas lives on immigrant labor," said Jeff Nielsen, executive vice president of the Houston Contractors Association. "Our economy is the way it is partly because cost of living is cheap and the reason for that is labor is cheap."

Throughout his presidential campaign, Donald Trump advocated a "deportation force" to track down and remove millions of immigrants here illegally. This week, he moved closer to that goal with a memo instructing federal authorities to broaden the scope of targeted deportations.

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The president's actions dovetail with a current push in the Texas Legislature to outlaw so-called sanctuary cities, requiring local law enforcement to cooperate with federal authorities on immigration enforcement.

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On Friday, the U.S. Hispanic Contractors Association and its Austin-based Texas arm sent a letter to Gov. Greg Abbott, warning that immigrants in Austin have been wary of showing up to work after an escalation of Immigration and Customs Enforcement activity.

"Our fear is that because of the perception that the public has on what the elimination of sanctuary cities means," the contractors wrote, "it will be difficult to find and retain experienced workers, which is especially damaging to small businesses."

Labor tight already

When construction slumped after the financial crisis, laborers lost their jobs and out of need for survival, they found other ones in other sectors. Now that construction is returning to life, the experienced workers are hard to get back.

"We do have a workforce shortage in this country for middle-skill workers in construction," said Jerry Nevlud, president of the Associated General Contractors of America Houston chapter. "The people who say the (immigrants here illegally) are taking jobs, you kind of wonder how many of them would have their sons and daughters get in the industry in its current state."

The depth of immigrants illegally here working in construction has been verified in countless studies. In 2013, the Workers Defense Project and the University of Texas at Austin surveyed 1,194 laborers on Texas work sites and found half were undocumented. A Pew Research report from November concluded that 28 percent of construction workers were undocumented, as well as 26 percent of agricultural workers and 17 percent of production workers.

In all, Texas was home in 2014 to about 1.7 million unauthorized immigrants - 24 percent working in construction, according to a study by the Waco-based Perryman Group. Those workers contributed $33.75 billion to Texas construction in 2015, out of a total gross product that year of about $85 billion.

That sort of economic impact can't be replaced, the study concluded. "Even if all currently unemployed persons filled jobs now held by undocumented workers, the state would be left with a glaring gap of hundreds of thousands of workers if the undocumented workforce were no longer available," the researchers wrote.

Texas builders say it's simply too difficult to recruit domestic workers, especially young adults, to consider construction work. Often, they are on the job for about two weeks - sometimes less in the summer - before they quit, said Mike Dishberger, owner of Sandcastle Homes and president of the Greater Houston Builders Association.

"Getting even high school kids, college kids to work outside in Houston heat, in the summer, is extremely hard," he said.

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Nestor Rodriguez, a sociologist who studied immigrant labor for years while at the University of Houston, was more blunt: "The commercial construction companies (say) only Mexicans stay on the roof when it gets to be 104 degrees."

On Friday, the five men building a cluster of townhomes in the Heights were Mexican. They agreed to talk about their work, but not their legal status. One used a forklift to raise a thousand-pound composite wooden beam and set it across the top of the two-story townhome's wooden frame, where the roof would eventually go. The others, standing on the floor below, would carefully scoot it across the length of the roof, using smaller wooden planks to push it.

"This is very dangerous work," said Ramón, a foreman from San Luis Potosí who has worked construction in Texas since 1994. He asked for his last name to be withheld from publication. "Our hands go for very cheap."

Hard work a necessity

Lupe de Leon, an undocumented laborer, has made a career in Texas construction. He grew up in Reynosa, Tamaulipas, crossing the Rio Grande before sunrise daily to pick tomatoes, lettuce, cantaloupe, broccoli or oranges in McAllen.

Back then, he said, it was common for Mexican workers to cross the border daily. He got work pouring in concrete in South Texas in the 1990s, and moved to Houston in 1996 to pursue work opportunities and to follow the woman who is now his wife and the mother of his two teenagers.

Once in Houston, he took on all sorts of construction work - ceilings, painting, framing houses, demolitions and Sheetrock, eventually becoming foreman for a construction company.

"When you have a necessity you learn to do whatever you must. Every time they offer work, you see opportunity," he said. "Difficult is one word that doesn't exist in the Latino community."

On most Houston work sites, he estimated, 90 percent of workers are Latino immigrants, and about half of them are unauthorized to be in the country. "The companies don't want to know if your green card or social security card is fake, they just need someone to do the job," he said.

Now de Leon owns a small company that hangs Sheetrock, plus four homes as investment properties. He sends money weekly to his parents, who he hasn't seen in 18 years because he can't safely leave and re-enter the country. He is getting ready to send his 17-year-old daughter to college.

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And yet, de Leon is nervous given his status and current events. He said his best friend got deported last week while going to pick up a colleague who had previously been deported. Officers with U.S. Immigration and Custom Enforcement were waiting outside the man's house.

If that were to happen to de Leon, his family would lack any financial support, and his eight employees would be out of work.

"I have a lot to lose," he said. "A lot of families are going to be destroyed."

Long-needed resolution

Such uncertainty can create all sorts of problems for the construction industry. Many contractors have already committed to large projects years in advance, trusting a reliable workforce won't be disrupted. That's why virtually all associations of builders or contractors have long supported federal immigration reform that would allow Mexicans and others to work in the U.S. without fear of deportation.

"We've been dealing with this for a long time, and we just need to get a resolution," said Nevlud with the General Contractors Association. "Congress refuses to move."

Efforts at comprehensive reform have stalled repeatedly, most recently under the Obama administration, and has been wiped from the agenda under Trump, whose stated goal is to remove immigrants living here illegally from the country. Proponents of hard-line immigration policy have argued that unauthorized workers should simply attain legal status, but experts contend that there is no such option for the class that builds Texas.

"The ability for these workers to come in legally for a temporary work program is about as close to zero as you can get," said Charles Foster, a veteran Houston immigration lawyer who advised on immigration policy for the George H.W. Bush administration. "There is no line to get legal. It's all a myth."

The closest thing, he said, was the H-2B visa program for temporary non-agricultural workers, which allows in about 66,000 people across the 50 states each year - hardly enough to account for the hundreds of thousands of laborers in Texas.

Now as construction picks up in many sectors after a multiyear slump, industry observers wonder what could happen if demand for workers grows while supply stays tight or shrinks.

Michael Berman, an independent financial adviser and fellow at Harvard University's Joint Center for Housing Studies, cited recent conversations with construction CEOs who worry that the Trump administration's plans for a trillion dollars in infrastructure spending could increase demand for labor while the labor pool shrinks due to tightening immigration rules. That would drive contractors into competition for available workers, pulling workers from across all construction sectors.

"That would be inflationary," Berman said. "And that would end up being passed on to the costs of homes."