President Trump is weighing allowing the release of the second of two memos addressing allegations of improper conduct by the FBI. The latest classified memo, drafted by Rep. Adam Schiff, the ranking Democrat on the House intelligence committee, allegedly defends the agency in a rebuttal to a memo that was released last week. The earlier memo was written at the direction of the committee chairman, California Republican Devin Nunes, and criticized the FBI’s surveillance methods in the early part of the Russia investigation.

The tussle over the two memos is leaving many observers with a sense of political whiplash. Democrats who were once quick to castigate the FBI and other intelligence agencies for overreaching on surveillance are now defending the agency’s need for secrecy. Meanwhile, Republicans like Nunes — who led the charge just a few months ago to pass legislation extending the government’s surveillance powers — are arguing that agents abused their authority.

The FBI certainly isn’t new to political controversy, but historians say that the current discord reflects an unprecedented willingness of legislators in both parties to use a crucial piece of the nation’s law-enforcement apparatus as a political football. And even if Democrats and Republicans return to their previous positions after the Russia investigation is over, these historians warn that the strife could permanently harm Congress’s relationship with the FBI.

Partisan opinion on the FBI has shifted dramatically

The swing in opinion on the FBI isn’t confined to Capitol Hill. Over the past 15 years, Democratic and Republican voters’ views of the agency have veered in opposite directions. In 2003, Gallup asked respondents to rate the job being done by the FBI and found that 63 percent of Republicans said the agency was doing an “excellent” or “good” job, while only 44 percent of Democrats agreed. Last year, however, Gallup asked the same question and found that the tables had turned: Only 49 percent of Republicans said that the FBI was doing an “excellent” or “good” job, compared to 69 percent of Democrats.

The polling data suggests that these changes were driven by different factors for each party, at least initially. Democratic approval of the FBI was considerably higher in 2009 than in 2003 — suggesting that the election of Barack Obama as president may have increased liberals’ confidence in the agency — while Republicans’ support for the agency held steady through 2014, then abruptly fell when the question was asked again in 2017.

Separate polling conducted by Pew, which tracks whether Americans have favorable or unfavorable opinions of an array of government institutions, found that Democrats’ outlook on the FBI grew considerably rosier over the course of the Obama administration. Pew’s findings for Republicans show a more muted decline in support for the FBI than Gallup’s: The percentage of Republicans who said they had a favorable view of the FBI dropped from 71 percent in 1997 to 65 percent in 2017.

Douglas Charles, a Pennsylvania State University history professor who studies the FBI, said that the Gallup results are evidence that Americans’ views of the law-enforcement agency are another casualty of today’s hyper-partisan rhetoric. “This isn’t about substance — the Republicans have decided that it’s politically expedient to attack the FBI,” he said. “And the Democrats have made a similar calculation that it’s politically expedient to support it.”

The FBI wasn’t always a political lightning rod

Although J. Edgar Hoover, the director of the FBI from 1924 to 1972, is well-known today for pursuing extensive surveillance against suspected communists, civil rights leaders and anti-war activists, the bureau received bipartisan support through the early 1970s, according to historians like Charles. It wasn’t until after Hoover’s death that the abuses of power under his watch were revealed and the agency became more politically divisive.

Founded in 1908 under a vague mandate from President Teddy Roosevelt’s attorney general, the authority and scope of the group of special agents that became the FBI remained limited until it was given significant new powers and funding by President Franklin D. Roosevelt, who authorized several secret investigations of American Nazi, fascist and communist groups in the 1930s. Around the same time, the agency raised its profile by significantly stepping up its efforts against organized crime.

Investigating gangsters, bank robbers, fascists and agitators during World War II was not a contentious mission, according to Kenneth O’Reilly, an emeritus professor of American political history at the University of Alaska. And the FBI’s activities remained mostly uncontroversial in the 1950s, when agents stepped up their scrutiny of suspected communists. At the time, O’Reilly said, Democratic politicians backed the FBI’s activities in an effort to mitigate the impact of the anti-communist investigations in Congress led by Republican Sen. Joseph McCarthy and the House Un-American Activities Committee by emphasizing the professionalism of the FBI. “They said essentially, ‘We agree that we need to root out communists, but these people in Congress are amateurs — we need the real pros, the FBI, to do this,’” O’Reilly said.

A few Democrats in Congress began to speak out against the FBI’s excessive use of wiretapping in the 1960s, but the tide didn’t start to turn until Hoover’s death in 1972 ended his tenure, according to David Garrow, a former professor of law and history at the University of Pittsburgh School of Law. The FBI came under additional scrutiny when it was revealed that Hoover’s successor had destroyed, on orders from President Richard Nixon’s administration, files owned by the organizer of the Watergate break-in.

In 1975, both houses of Congress launched investigations that exposed widespread illegal spying on American citizens and foreign leaders in the name of national security, including civil rights leaders like Martin Luther King Jr. “The whole tenor of the conversation about the FBI changed — there was less deference and more suspicion, particularly from Democrats,” Garrow said.

Republicans, Garrow added, continued to protect the FBI, particularly under Ronald Reagan, who pardoned two FBI officials (including W. Mark Felt, later revealed to be Watergate informant “Deep Throat”) who were convicted of violating Americans’ constitutional rights in pursuit of fugitive anti-war activists.

Today’s strife between Congress and the FBI is unprecedented — and could cause lasting damage

Historians like Garrow say that in light of of this history, the Democrats’ current support for the FBI’s tactics is just as surprising as the Republicans’ sudden opposition. In an ordinary political universe, Garrow said, Democrats would be more likely to be critical of the FBI’s handling of the surveillance warrant at issue in the dueling memos. (The Nunes memo alleges that the FBI improperly concealed information about the possible political bias of the material agents used to request a warrant to wiretap “informal” Trump campaign adviser Carter Page.)

“Normally you might expect Democrats to raise some questions about long-term surveillance of a U.S. citizen who seems like a pretty minor actor in the Trump campaign,” he said. “But instead, anything that seems to be anti-Trump or damaging to Trump is being embraced without question.”

This is troubling, Garrow continued, because “we should have the same standard for whether surveillance is acceptable for any citizen, whether it’s Carter Page or Martin Luther King.” The Democrats’ willingness to abandon that standard when it’s politically convenient, he said, could amount to an abdication of their traditional role as FBI watchdogs.

Charles said that he was similarly surprised by the Republican offensive against the FBI, which has been led by President Trump. “The FBI is under attack right now by people who have historically been its biggest boosters,” Charles said.

Charles added that the GOP’s sudden willingness to release classified information in the Nunes memo — whose publication the FBI publicly opposed — might undermine the relationship between Congress and the intelligence community. One of the lessons of the Hoover era was that the FBI needed congressional oversight; the House and Senate intelligence committees were created for exactly this purpose. But this oversight will be much less effective, Charles warned, if the FBI stops its voluntary participation. “That may lead [FBI agents] to conclude going forward that they shouldn’t share information that the FBI really needs to be sharing,” he said.

And even if Democrats and Republicans later return to their old battle lines, the current imbroglio signals the depth of the country’s partisan animosity. “I can’t believe that I’ve lived to see liberal Democrats wholeheartedly embracing the FBI’s tactics,” said David Stebenne, a professor of American political and legal history at Ohio State University. “It’s really something when Democrats clearly perceive that the FBI is more credible and trustworthy than the president.”