John McCain. Jonathan Ernst/Reuters Republican Sen. John McCain invoked the biggest scandal in modern political US history on Tuesday when finding a comparison for recent reports about President Donald Trump.

"We've seen this movie before. It's reaching Watergate size and scale," McCain said at a dinner for the International Republican Institute. "This is not good for the country."

The New York Times reported on Tuesday that Trump in February had asked James Comey, then the FBI director, to end the FBI's investigation into national security adviser Michael Flynn.

Trump unexpectedly fired Comey last week, in a move Trump acknowledged making at least in part because of Comey's handling of the FBI investigation into whether the Trump campaign colluded with Russia to meddle in the 2016 election.

Several Democratic lawmakers called the dismissal "Nixonian" and compared it to the Saturday Night Massacre of 1973, when Nixon ordered the firing of the special prosecutor investigating the Watergate break-in and cover-up. Nixon's approval rating never recovered, and he resigned in disgrace the following year.

McCain has been consistently critical of Trump since Comey's firing, and he is one of just a handful of congressional Republicans who have called for the Russia investigation to be handled independently.