The New York Democrat's comments come after news reports
that Trump requested Comey pledge his loyalty to the President at a dinner shortly after Trump took office. Nadler's remarks also follow the President's comment in an interview with NBC's Lester Holt on Thursday that he considered
the FBI's investigation of alleged collusion between Trump campaign aides and Russian officials in his decision to fire the FBI director.
"Asking for loyalty of the FBI, which is supposed to be independent -- especially when he just admitted he was thinking of firing Comey essentially because of the Russian investigation -- those two things combined make a very strong case for the President having committed an obstruction of justice and that's got to be investigated," Nadler said on CNN's "Erin Burnett OutFront."
On Friday, Trump tweeted: "James Comey better hope that there are no 'tapes' of our conversations before he starts leaking to the press."
The President's reasoning for dismissing Comey, which Trump told NBC News
was something he had been planning to do before it was recommended by his attorney general and deputy attorney general, "raises a very serious concern about obstruction of justice, about obstructing that investigation," Nadler said.
Other US officials, including prominent GOP lawmakers such as Sen. John McCain, have said they were "troubled" by Trump's abrupt firing of Comey mid-investigation. Many Democratic lawmakers are calling for the appointment of a special prosecutor to take Comey's place leading the probe.
When he fired Comey earlier this week, Trump garnered comparisons to President Richard Nixon and his infamous decision to remove the special prosecutor investigating Watergate crimes in 1973.
Earlier Friday, Democratic Reps. Elijah Cummings and John Conyers, the respective ranking members of the House oversight and judiciary committees, requested from White House counsel Donald McGahn copies of all recordings between Trump and Comey.
In a letter, the two lawmakers also said Trump's actions "raise the specter of possible intimidation and obstruction of justice."
Still, Nadler said on CNN that it's still "too early" to talk about impeachment as an avenue Democrats would pursue.
On Wednesday, Sen. Richard Blumenthal said on "Anderson Cooper 360"
that impeachment proceedings are a possibility, but he agreed that that would be a ways down the road.
"It may well produce impeachment proceedings, although we're very far from that possibility," the Connecticut Democrat said.
Like some lawmakers, including California Democratic Sen. Dianne Feinstein, Nadler said he believes that if Trump did record conversations with Comey, which the President's tweet suggested he did, then those recordings would be government property.
"I don't know whether he taped those conversations," Nadler said. "But if he did, the Nixon court precedent says those tapes are the property of the government, must not be destroyed and should be turned over."