WASHINGTON (AP) -- It's not really about the books.

President Donald Trump's abrupt firing of top officials at the Library of Congress and equally sudden attempt to appoint a slate of loyalists as replacements has instead morphed into an enormous fight over the separation of powers, as the White House tries to wrest control of what has for centuries been a legislative institution.

It's a power struggle with potentially vast consequences. The Library of Congress not only stores the world's largest collection of books but also an office overseeing reams of copyrighted material of untold value.

There is a research institute that has long been protected from outside influence. Its servers house extremely sensitive information regarding claims of workplace violations on Capitol Hill, as well as payments and other financial data for the legislative branch's more than 30,000 employees. There's even speculation that the whole affair is tied to an ongoing debate over whether big tech companies can use copyrighted material for artificial intelligence systems.

Because of this, the battle over control of the Library of Congress has prompted Republican leaders on Capitol Hill to deliver rare pushback against a president who has pressed to expand the boundaries of his own power to enact his priorities. Senate Majority Leader John Thune and other Republicans have been talking with the White House about a potential path to détente.

This all has left the library in a bizarre state of purgatory.

For now, Trump's choices for interim library leaders -- most notably Todd Blanche, a deputy attorney general who had represented the president in his criminal proceedings -- have not appeared to challenge the assertion by the library that one of its veteran officials would be the acting head. It would be unheard of for an executive branch official such as Blanche to simultaneously serve in the legislative branch, according to experts.

"This egregious overreach into the legislature by the executive branch is just unwarranted and, we believe, unprecedented," said New York Rep. Joe Morelle, the top Democrat on the House Administration Committee, which oversees the Library of Congress.

The controversy began to unfold publicly last week, when Librarian of Congress Carla Hayden was fired in a terse email from a White House official. She had just one year left in her 10-year term.

Then this week, Blanche was tapped by the White House to be the interim librarian, and two other Justice Department officials were chosen for other senior library posts. Those officials, Brian Nieves and Paul Perkins, tried unsuccessfully to enter the U.S. Copyright Office on Monday, but left voluntarily after library officials called Capitol Police.

Thune told The Associated Press that Congress was "not entirely" consulted ahead of Trump's dismissal of Hayden.

Lawmakers want to ensure that "congressional legislative branch equities are protected," Thune said. He speculated that discussions with the White House to resolve the standoff would bleed into next week.

The White House has said Trump was within his authority to dismiss Hayden, a former head of the library systems in Baltimore. It cited "quite concerning" behavior from Hayden involving diversity, equity and inclusion efforts as well as books for children that the White House found inappropriate.

A copy of virtually every book published in the U.S. is sent to the Copyright Office, then the library decides whether to include it in the main collection. No one under 16 can get a reader card to access the collection.

Existing regulations and past practices call for an acting librarian to come from the Library of Congress' current ranks if there is a vacancy. But the White House has argued that a law governing federal vacancies applies, even though the 1998 law deals with the executive branch, according to two people familiar with the discussions.

When Nieves and Perkins showed up on Monday, they held a letter that invoked the vacancies law to justify their appointments, according to one of the people.

Oklahoma Sen. Markwayne Mullin, who leads the panel overseeing funding for the Library of Congress, argued that in practice, the librarian is not a legislative branch employee, saying: "It's an appointment by the president of the United States, because we have to confirm her."

Yet there is deep concern among lawmakers and aides about any unjustified intrusion of the administration into the Library of Congress and its operations.

Especially worrisome to them is potential meddling with the Congressional Research Service, known as the nonpartisan think tank of Capitol Hill. It fields roughly 75,000 requests from members of Congress every year for research, legal expertise and other information critical for policymaking.

The discussions between lawmakers and the Congressional Research Service are considered so sensitive that they are protected under the speech or debate clause of the Constitution, which shields members of Congress from being questioned -- such as in court -- about official legislative acts.

The service's "utility and trustworthiness would be substantially undercut if these inquiries were not protected or the Administration sought to shape responses to reflect its priorities," said Hope O'Keeffe, a former associate general counsel at the Library of Congress.

The library also oversees the Office of Congressional Workplace Rights, which functions as the human resources office of the legislative branch, fielding complaints about harassment, discrimination and other workplace violations. It also stores financial information about legislative branch employees, who include not just those at the Library of Congress, lawmakers and their aides, but employees of the Capitol Police, the Architect of the Capitol and the Government Accountability Office.

Robert Newlen, the principal deputy librarian, told library staffers shortly after Hayden's firing that he will serve as the acting librarian. He said in a note this week that while the White House had appointed its own acting librarian, "we have not yet received direction from Congress about how to move forward," indicating that the library was defying Trump's wishes.

California Sen. Alex Padilla, the top Democrat on the Senate Rules Committee, this week said flatly that Newlen is the acting librarian of Congress. Asked whether Blanche was respecting that, Padilla said, "that's my understanding."

But a White House official stressed on Thursday that Trump selected Blanche to be the acting librarian. The official, granted anonymity because of the ongoing private discussions with lawmakers, said Trump chose Blanche because the president is "appointing highly-qualified individuals who are wholeheartedly committed to advancing the America-first agenda." The Justice Department did not respond to a request for comment.

Some suspect that the Copyright Office is the true aim of the administration. Housed in the Library of Congress with a leader chosen by the librarian, the office accepts millions of copies of copyrighted material such as books, artwork and music every year as part of the copyright registration process.

Shortly before the director of the office, Shira Perlmutter, was fired, her office released a report that questioned whether it was legal for the tech industry to use copyrighted material to "train" their artificial intelligence systems. Tech companies contend that doing so is legal when used for educational or research purposes or creating something new. Perlmutter's report said doing so, in some circumstances, would go beyond established boundaries of fair use when the AI-generated content is competing with creative works made by people.

The material there is extremely valuable. For instance, copyright violation damages for the office's existing collection -- if, for instance, tech companies scraped the material for AI purposes and then later were found liable for copyright infringement -- would likely exceed $1.5 trillion, according to a person familiar with the calculations.

Morelle noted that the firing came "one day after, and I doubt there's any coincidence to this, a report which is in many ways at odds with what Elon Musk wants to do around intellectual property and copyright." Musk is the billionaire outside adviser for Trump who operates his own AI startup, called xAI. The White House did not respond to a request for comment.

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Associated Press writer Alanna Durkin Richer contributed to this report.