President Donald Trump's surprise firing of FBI Director James Comey has drawn comparisons to the Nixon-era "Saturday Night Massacre".
Both cases involve a president getting rid of an official leading an investigation that could ensnare the White House.
On that Saturday night in 1973, Nixon ordered the firing of the independent special prosecutor overseeing the Watergate investigation, prompting the resignations of the top two officials at the Justice Department.
This week, Trump fired the FBI director in the midst of an investigation into whether Trump's campaign had ties to Russian meddling in the election that may have helped send him to the White House.
Douglas Brinkley, a presidential historian at Rice University, said the comparison was "apt".
"Obviously there are different circumstances but it's about a president that's seeming to lurch into abuse of power," he said.
Nixon ordered Archibald Cox fired for his continued efforts to obtain tape recordings made at the White House. Cox had said he would not bow to "exaggerated claims of executive privilege" and drop his pursuit of the tapes.
Attorney General Elliot Richardson refused to carry out Nixon's order and resigned in protest.
Richardson's deputy, William Ruckelshaus, also refused and resigned as well. Finally, Solicitor General Robert Bork, the third-ranking official at Justice, fired the prosecutor.
In this case, Trump had the power to fire the FBI director himself. The White House cited a Justice Department official's concerns about the director's handling of last year's investigation into Hillary Clinton's email practices.
But Democrats criticising Trump's stunning move say the two cases are similar because Comey was overseeing an FBI investigation into both Russia's hacking of Democratic groups last year and whether Trump campaign associates had ties to Moscow's election interference.
Three US officials say Comey told members of Congress he had recently asked the Justice Department for more money for the bureau's investigation into Russia's election meddling.
"This is Nixonian," Democrat Senator Bob Casey declared on Twitter on Tuesday, calling for a "special prosecutor to continue the Trump/Russia investigation".
The White House has said there is no evidence of any ties between the Trump campaign and Russia.
In his letter to Comey, Trump stressed the FBI director had told him "on three occasions, that I am not under investigation".
There are some differences. Brinkley noted that the Watergate investigation was further along, while the Russia probe is just beginning. And after Nixon's firings, Brinkley said a number of Republicans "started going after the leader of their own party" and that has not happened yet in Trump's case.
