WASHINGTON — Snitches, moles, spies, whistleblowers. Government informants are an age-old investigative tool that’s as much a part of the FBI’s 110 years of history as J. Edgar Hoover or its “10 Most Wanted” list.

In the case of President Donald Trump, the FBI called on a longtime informant — identified by several news outlets as an American professor living in Britain — to ascertain whether Trump’s campaign aides accepted help from the Russian government to sink Hillary Clinton’s presidential ambitions. That jury is still out, with a special counsel appointed to investigate.

In the meantime, Trump and closely aligned Republicans in Congress have flipped the tables on the politically damaging Russia probe by calling for a new investigation — this time into whether the FBI spied on his presidential campaign in its own bid to sway the 2016 election.

“If the FBI or DOJ was infiltrating a campaign for the benefit of another campaign, that is a really big deal,” Trump wrote last weekend in a tweet punctuated with his campaign slogan: “Drain the Swamp!”

CRAZY? MAYBE NOT.The FBI has successfully investigated big-city mobsters, the Ku Klux Klan and domestic terrorists. But it has also probed the work of Rev. Martin Luther King Jr., Malcolm X and the Beatles’ John Lennon.Since Hoover’s death, the FBI has enacted several reforms including 10-year term limits on its director and new rules about domestic investigations intended in part to insulate the agency from politics.

HAVING SAID THAT ...The mere existence of a government informant in an investigation doesn’t mean a probe is tainted. It’s a legal and widely accepted practice that’s hardly a secret. And Trump’s accusation that the FBI “planted” a source on his campaign doesn’t stand up to scrutiny.The agency itself addresses the practice on its web site, noting “special care is taken to carefully evaluate and closely supervise their use so the rights of individuals under investigation are not infringed.”Informants “may receive compensation in some instances for their information and expenses,” the FBI writes, but they aren’t considered employees.The agency seemed acutely aware of the political pitfalls of investigating Trump’s campaign, keeping it under wraps in its early stages and, according to reports, sending a longtime source to question lower level aides.

WHERE WE GO FROM HEREIn Trump’s corner are several House Republicans who are demanding access to the FBI’s closely guarded secrets in the Russia probe, including details on the Russia informant.“Let’s cut through the recalcitrant bureaucracy, get the truth, and hold people accountable!” tweeted GOP Rep. Ron DeSantis of Florida.Top FBI and Justice Department officials have already agreed to meet with congressional leaders and “review” highly classified documents in the case. Also, Deputy Attorney General Rod Rosenstein said the Justice Department’s inspector general will look into whether any surveillance was politically motivated.

THE BOTTOM LINEConservative pundits say liberals would be outraged had the FBI put Obama’s campaigns under surveillance. Liberal pundits counter that conservatives would be outraged if Clinton’s campaign aides sought or accepted the help of the Russian government ahead of the election, and the FBI ignored it.All of this puts pressure on special counsel Robert Mueller to conclude his investigation in what has become the most politically charged atmosphere in Washington in decades.“Spying on campaigns is extraordinary. Mueller better have the goods,” tweeted Ari Fleischer, the White House press secretary under President George W. Bush, on Tuesday.