Democrats have called for an investigation into whether the U.S. president puts pressure on the IRS to deprive Harvard of status.

U.S. President Donald Trump restored his threat to Harvard’s tax-free status, a move that could cost Ivy League institutions billions of dollars in implementation.

"We will take Harvard's tax-free status. It's what they deserve!" Trump wrote in a post on Friday on his social media platform Truth Social.

The statement is the latest threat to the ongoing dispute between the Republican president and Harvard University, the country's oldest higher education institution.

Since taking office on January 20, Trump has been trying to increase control over U.S. universities and crack down on what he believes is "illegal protest" on campus.

But Harvard has been the focus of his public anger, especially after school leaders rejected a list of demands issued by the Trump administration, issued on April 11.

In a five-page letter (PDF), Trump officials called on Harvard to reform its student discipline system, investigate protesters involved in pro-Palestinian demonstrations, commission an external audit of the program considered anti-Semitism, and eliminate its diversity, equity and inclusion program (DEI) program.

The letter also calls for Harvard to commit to “diversity of perspectives” in its hiring and admissions practices, which is achieved through “structural and personnel change.” The task is a dispute Trump has tried to implement a political touchstone test on schools, critics say.

By April 14, Harvard President Alan Garber rejected the letter. In a statement posted on the Harvard website, he explained that Trump's demands violated Harvard's academic freedom, as well as other rights.

“Universities will not give up independence or waive their constitutional rights,” Garber wrote. “No government has a government—whoever the party in power should decide what private universities can teach, who they can acknowledge and hire, and what areas of study and inquiry they can pursue.”

The Trump administration responded quickly, suspending nearly $2.2 billion in grants and contracts to schools.

It has since filed a lawsuit (PDF), challenging Trump's attempt to withhold federal funds from schools, saying the move "leverage to control academic decisions at Harvard."

Shortly after stopping grants and contracts, Trump turned his attention to Harvard’s tax-free status, exacerbating pressure on schools.

Educational institutions, charities, nonprofits and institutions that are scientifically researched are entities that are eligible to apply for tax exempt status under U.S. law. This status exempts these organizations from paying federal income and property taxes.

But on April 15, Trump was addicted to Harvard University on social media, which should have lost its waivers as it handles protests from pro-Palestinian students.

Trump wrote: "Perhaps Harvard should lose its tax-free status if it continues to push political, ideology and terrorist inspiration/support for "disease?" ”, then it should be taxed as a political entity.”

The U.S. media report later showed that despite relatively few such actions, the Internal Tax Services Agency (IRS) is still considering internally whether to revoke the school’s IRS status.

But under the U.S.C., the illegal acts of the president, vice president or any of his employees are illegal, and even indirectly request an audit of any taxpayer from the IRS.

Democrats seized on this fact to promote an investigation into the Trump administration’s actions toward Harvard.

“The president is targeting Harvard’s nonprofit status to achieve blatant political purposes,” Democratic senators including Chuck Schumer, Ron Wyden and Elizabeth Warren wrote in a joint letter Friday.

“The president’s call for Harvard to lose his tax-free status raises disturbing constitutional questions, including whether the president is trying to squeeze Harvard’s right to freedom of speech and whether revoking its tax-free status would deprive the university of due process rights.”