Sanders cited the testimony of Comey last week before Congress, where he detailed how he had decided to hold a news conference about the FBI probe into Hillary Clinton's email server - and did that without the support of the Attorney General, Loretta Lynch.
"Frankly, I think it's startling that Democrats aren't celebrating this, since they've been celebrating for so long," Sanders added.
Earlier on Capitol Hill, Vice President Mike Pence made clear his strong support for Mr. Trump's decision to fire the FBI chief.
"The President made the right decision at the right time," Pence told reporters, as he emphasized a main argument from the White House in recent days.
"There is no evidence of collusion between our campaign and any Russian officials," the Vice President added, even though a formal investigation into that question remains underway.
In Congress, most Republicans stuck with the President.
"President Trump acted decisively and within his authority, and I stand behind him," said Sen. David Perdue (R-GA).
"It was the right thing to do," said Sen. Jim Inhofe (R-OK), who said Comey has "changed his position so many different times, on everything."
"His politicization of the Clinton email scandal made it hard for him to run the FBI," said Sen. Rand Paul (R-KY) of Comey.
But there were other Republicans who felt the timing and the appearance of the Comey decision - with an ongoing FBI investigation that seems to be touching the White House - did not make for a good appearance.
"A little surprised in the timing," said Sen. James Lankford (R-OK), who told me he was "trying to find out why this was the moment" that Comey needed to be fired.
"I believe the White House should provide a fuller explanation regarding the President's rationale," said Sen. Rob Portman (R-OH).
As for Democrats, they hammered on their call for a special counsel investigation, and raised direct questions about whether the President had something to hide.
"What is happening now is the beginning of the appearance of a cover up," said Sen. Bill Nelson (D-FL), as Democrats made clear they don't believe Comey was booted because of how he handled the Hillary Clinton email scandal.
"I don't think anyone out there is going to believe that Donald Trump rose to the defense of Hillary Clinton in firing Director Comey," said Sen. Chris Murphy (D-CT).
But without some support from Republicans to force votes on a special counsel probe and more, Democrats could only voice their frustration and concern over what will happen to the probe of possible Trump-Russia ties.
"This is an obstruction of justice by the Trump administration masquerading as a personnel decision," said Sen. Ed Markey (D-MA).
Meanwhile at the White House, the President met today with top Russian officials, including the foreign minister, and the Russian Ambassador to the U.S., whose phone calls caused trouble for ex-National Security Adviser Michael Flynn.
One note about the pictures from the Oval Office of these meetings - no reporters of photographers who cover the White House were allowed in, as the only pictures came from Russia - that's why this tweet came from the Russian embassy.