WASHINGTON (AP) -- Three decades ago, foreign students at Harvard University accounted for just 11% of the total student body. Today, they account for 26%.

Like other prestigious U.S. universities, Harvard for years has been cashing in on its global cache to recruit the world's best students. Now, the booming international enrollment has left colleges vulnerable to a new line of attack from President Donald Trump. The president has begun to use his control over the nation's borders as leverage in his fight to reshape American higher education.

Trump's latest salvo against Harvard uses a broad federal law to bar foreign students from entering the country to attend the campus in Cambridge, Massachusetts. His order applies only to Harvard, but it poses a threat to other universities his administration has targeted as hotbeds of liberalism in need of reform.

It's rattling campuses under federal scrutiny, including Columbia University, where foreign students make up 40% of the campus. As the Trump administration stepped up reviews of new student visas last week, a group of Columbia faculty and alumni raised concerns over Trump's gatekeeping powers.

"Columbia's exposure to this 'stroke of pen' risk is uniquely high," the Stand Columbia Society wrote in a newsletter.

People from other countries made up about 6% of all college students in the U.S. in 2023, but they accounted for 27% of the eight schools in the Ivy League, according to an Associated Press analysis of Education Department data. Columbia's 40% was the largest concentration, followed by Harvard and Cornell at about 25%. Brown University had the smallest share at 20%.

Other highly selective private universities have seen similar trends, including at Northeastern University and New York University, which each saw foreign enrollment double between 2013 and 2023. Growth at public universities has been more muted. Even at the 50 most selective public schools, foreign students account for about 11% of the student body.

America's universities have been widening their doors to foreign students for decades, but the numbers shot upward starting around 2008, as Chinese students came to U.S. universities in rising numbers.

It was part of a "gold rush" in higher education, said William Brustein, who orchestrated the international expansion of several universities.

"Whether you were private or you were public, you had to be out in front in terms of being able to claim you were the most global university," said Brustein, who led efforts at Ohio State University and West Virginia University.

The race was driven in part by economics, he said. Foreign students typically aren't eligible for financial aid, and at some schools they pay two or three times the tuition rate charged to U.S. students. Colleges also were eyeing global rankings that gave schools a boost if they recruited larger numbers of foreign students and scholars, he said.

But the expansion wasn't equal across all types of colleges -- public universities often face pressure from state lawmakers to limit foreign enrollment and keep more seats open for state residents. Private universities don't face that pressure, and many aggressively recruited foreign students as their numbers of U.S. students stayed flat. The college-going rate among American students has changed little for decades, and some have been turned off on college by the rising costs and student debt loads.

Proponents of international exchange say foreign students pour billions of dollars into the U.S. economy, and many go on to support the nation's tech industry and other fields in need of skilled workers. Most international students study the STEM fields of science, technology, engineering and math.

In the Ivy League, most international growth has been at the graduate level, while undergraduate numbers have seen more modest increases. Foreign graduate students make up more than half the students at Harvard's government and design schools, along with five of Columbia's schools.

The Ivy League has been able to outpace other schools in large part because of its reputation, Brustein said. He recalls trips to China and India, where he spoke with families that could recite where each Ivy League school sat in world rankings.

"That was the golden calf for these families. They really thought, 'If we could just get into these schools, the rest of our lives would be on easy street,'" he said.

Last week, Trump said he thought Harvard should cap its foreign students to about 15%.

"We have people who want to go to Harvard and other schools, they can't get in because we have foreign students there," Trump said at a news conference.

The university called Trump's latest action banning entry into the country to attend Harvard "yet another illegal retaliatory step taken by the Administration in violation of Harvard's First Amendment rights."

In a lawsuit challenging the Trump administration's previous attempt to block international students at Harvard, the university said its foreign student population was the result of "a painstaking, decades-long project" to attract the most qualified international students. Losing access to student visas would immediately harm the school's mission and reputation, it said.

"In our interconnected global economy," the school said, "a university that cannot welcome students from all corners of the world is at a competitive disadvantage."

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