Since declaring his candidacy last June, President-elect Donald Trump's promise to build a border wall on Mexico's dime has become one of the rallying cries of his campaign, drawing cheers from some crowds and boos from others.

But it's Mr. Trump's stances on legal immigration reform that have tech industry leaders in Silicon Valley concerned for the future.

Trump's feelings toward the H-1B visa program for temporary foreign workers – the majority of which work in technology fields – appear to have reversed course throughout his campaign, raising anxiety among industry leaders who say there aren't enough skilled Americans to fill those jobs.

After initially proposing that the H-1B program be restricted, that tech companies be required to hire Americans first, and that the prevailing wage for H-1B workers be raised, Trump appeared to have changed his mind, saying during a Republican debate, "We need highly-skilled people in this country and if we can't do it, we'll get them in." Shortly after, he put out a statement promising to "end forever" the H-1B program, calling it a "cheap labor program."

The president-elect's stance on the program has remained largely unclear since, and the days following the election – during which he has surrounded himself with advisers and backers with strict views on immigration and said that sweeping changes to US immigration policy would rank among his top three priorities – have contributed to growing speculation and concerns that the tech industry may soon find itself short necessary workers.

Silicon Valley "would be the first to suffer if it got harder to come to America or if increasing xenophobia made fewer people want to," investor Paul Graham, co-founder of startup accelerator Y Combinator, told Bloomberg.

"If you’re a US tech company, your recruitment just got a lot harder," added Ava Benach, founding partner of Benach Collopy, an immigration law firm in Washington. "Finding people who want to come to the US now may be more difficult. People who have brown skin might feel pretty uncomfortable coming to the US under these circumstances."

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But Trump isn't the H-1B program's only critic, and there has been bipartisan support for reform, as Schuyler Velasco reported for The Christian Science Monitor in February.

Its supporters, including the Obama administration, have touted the H-1B as a way to inject highly sought-after skills into the US talent pool. But more often, critics say, big companies exploit it to reduce labor costs, leaving more and more skilled American tech workers out of a job. “There are literally tens of thousands of American workers who have trained their foreign replacements,” says Ronil Hira, a public policy professor at Howard University and a leading authority on H-1Bs. “And the workers being imported have no more than ordinary skills that are abundantly available in the US.” Mr. Hira believes the H-1B can be a boon for the US economy and for immigrants in search of a better life – his parents came to the US as skilled workers in the 1950s – but that it needs some serious revisions. And as the controversy over both H-1Bs and immigration writ large heats up, a growing force of economists and lawmakers, are offering up ways to do it, from requiring companies to prove they recruited in the US first to limiting the program to foreign workers with advanced degrees. Workers like [Leo Perrero, a former IT worker at Walt Disney World who was replaced by an Indian guest worker], meanwhile, are starting to speak up.

Critics of the H-1B program – and supporters of Trump – are harder to find among Silicon Valley executives. In July, nearly 150 current and former tech executives for firms including Apple, Facebook, eBay, Twitter, and Wikipedia signed an open letter in which they argued that the then-presumptive Republican nominee, who "traffics in ethnic and racial stereotypes," would be "a disaster for innovation," noting that 40 percent of Fortune 500 companies were founded by immigrants or their children.

"We believe that America’s diversity is our strength," the letter's signers wrote. "We also believe that progressive immigration policies help us attract and retain some of the brightest minds on earth – scientists, entrepreneurs, and creators."