President Trump wants fewer unauthorized immigrants. What is more ambiguous, yet more consequential, is his stance on legal immigration.

Does he want fewer authorized foreigners coming in – as some of his supporters are calling for – or does he simply want to change the current mix so fewer unskilled workers get admitted?

Either way, he faces a huge challenge. Immigration reform is complex and contentious. Even with both houses of Congress in his party’s control, Mr. Trump’s reforms will be complicated because they expose major divisions within the GOP. His moves will be closely watched because he made immigration – and standing up for working-class white voters – a central theme of his campaign.

After Sens. Tom Cotton (R) of Arkansas and David Purdue (R) of Georgia introduced a bill last month to cut by half the number of new permanent resident visas, or green cards, Trump had a nuanced response. He welcomed the bill, saying he wanted to see Congress pass a merit-based system overhaul that would favor “high skill” workers, according to Senator Cotton who met with Trump a week ago. “And that's exactly what we will do."

That’s not exactly an endorsement of Cotton’s drive to reduce overall immigration. Instead, Trump appears to want fewer low-paid immigrants – seen as competing for jobs with his most fervent base.

“Switching away from this current system of lower-skilled immigration, and instead adopting a merit-based system, we will have so many more benefits,” he told Congress on Feb. 28. “It will save countless dollars, raise workers' wages, and help struggling families, including immigrant families, enter the middle class. And they will do it quickly and they will be very, very happy indeed.”

Third time a charm?

Making such a sweeping change will prove difficult, if history is any guide. Similar proposals were part of blockbuster bills that Congress failed to pass in 2007 and 2013, largely because of GOP opposition to any form of amnesty for undocumented migrants.

Neither tried to reduce how many people could permanently immigrate per year. Instead, the 2007 bill, which President George W. Bush supported, restricted who could be sponsored for visas as a family member and set out a complex points system for admissions. It included points for education, English proficiency, and workplace skills. The 2013 Senate bill echoed this approach, ending preferences for siblings and adult married children.

Current US immigration policy, by contrast, is rooted in the idea of family unification: US citizens and permanent residents can petition to bring in relatives, from spouses and children to siblings and parents. Few countries are as generous in admitting extended families, says Marta Tienda, a professor of demographer and sociology at Princeton University in New Jersey.

Restricting family-based visas makes economic sense in the case of older parents and siblings given their expected health care costs, but such a move could prove unpopular, Ms. Tienda says. “Who wants to cut family visas? It’s like motherhood and apple pie.”

Another obstacle is the divide between establishment Republicans and cultural conservatives.

Businesses and some GOP lawmakers don’t want to choke off the global pipeline of people moving here because they see the economic upside. Since 1965, more than half of US population growth resulted from immigrants and their children and grandchildren, according to Pew Research Center.

Immigration also has a positive impact on economic growth, according to a 2016 National Academy of Sciences report. While first-generation migrants cost taxpayers more in terms of public services, as their children are educated and their incomes are lower, subsequent generations contribute far more in taxes than they get back. The same study identified a modest, short-term decline in the wages and employment of low-paid workers.

Moreover, immigrants who arrive without formal qualifications don’t stand still. “It’s an incredibly dynamic economy. People move up and gain skills,” says Tim Kane, a research fellow in immigration studies at Stanford University’s Hoover Institute, a conservative think tank. Immigrants are also more likely to start companies and hire other people than native-born workers.

Hitting the pause button

But some of Trump’s closest political allies oppose continued high immigration on cultural grounds. They say rising ethnic diversity threatens conservative values rooted in European heritage and Christianity. Steve Bannon, his chief strategist, has questioned why so many US tech CEOs are Asian and, as chairman of Breitbart, published numerous anti-immigrant and anti-Muslim stories. In 2015, Jeff Sessions, Trump’s attorney general, told Mr. Bannon in a Breitbart radio interview that a controversial 1924 law that imposed quotas on immigration and slowed new arrivals had been good for assimilation.

The idea: After a huge influx of immigrants, the US needs a pause in order to assimilate – just as it did after the great wave of immigration from the early 1880s to the 1920s. One in seven US residents is currently foreign-born, the highest in a century.

But such arguments can easily tip into justification for racial quotas.

"You cannot rebuild your civilization with somebody else's babies,” Rep. Steve King (R) of Iowa told CNN Monday, following up on a tweet from Sunday. It’s a message he says he’s told Europeans: “You've got to keep your birth rate up, and … you need to teach your children your values."

Some Republican legislators were quick to hit back: “Do I qualify as ‘somebody else's baby?’ " tweeted Rep. Carlos Curbelo (R) of Florida.

“Get a clue, @SteveKingIA,” tweeted US Rep. Ileana Ros-Lehtinen, also a Florida Republican. “Diversity is our strength.”

Of course, racial identity plays a role in Democrats’ immigration views, too, points out Mr. Kane of the Hoover Institute. Democratic politicians court Mexican and other Latino voters by standing up for family-based visas and trying to block reforms. Since Latinos are the largest immigrant community, they benefit from the current system that allows them to sponsor family members.

Tricky policy

Past administrations have adjusted immigration policy, adding new visa categories and tweaking different programs, says Daniel Kanstroom, a law professor at Boston College and co-director of its Center for Human Rights and International Justice. But that’s a long way from passing major legislation to change the selection process for immigration. “There have been adjustments made over the year but in terms of restructuring the whole immigration system it’s still largely oriented towards family,” he says.

What Trump will do is anybody’s guess. He says he wants to bring in high-skilled immigrants. But he also wants to stop abuses of visas that allow high-tech and other companies to bring in skilled workers when they can’t find Americans to do the job. The visas are temporary and subject to caps. But once here, the workers can try to obtain permanent status – and many do, says Tienda. “There’s nothing more permanent than a temporary worker.”

Immigration policy is tricky to get right.

Politicians need to respond to people’s fears about social cohesion and national security, Kane says. But “we’re getting it right way more than people appreciate” with the current policy.

Proponents of making US immigration policy more merit-based point to Canada and Australia as immigrant-friendly countries that use a points system to bring in skilled workers that compliment their economies.

But it wouldn’t necessarily produce more “PhD” immigrants for the US, says Mark Regets, a US-based research fellow at the Institute for Study for Labor in Bonn, Germany. “People move across borders for very complex reasons. And a lot of high-skill people move, in part, because of family connections,” he says.

A 2007 survey by the National Science Foundation of scientists and engineers who immigrated as adults to the US found that more cited family reasons than educational or job opportunities for making their move. This applies not just to scientists with existing kinship ties in the US but also the future prospect of being able to bring over their families and give them a better life.

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And the composition of immigrants might not change that much, either. In 2013, China and India passed Mexico as the largest country-of-origin for new immigrants, according to the Census Bureau. Given the high numbers of university graduates from India and China, merit-based selection would not slow their arrival.

“You would not necessarily be redirecting immigration to Europe by making it skills-based,” says Mr. Regets.