"I think that Comey acted in an outrageous way during the campaign," Sanders said. "No one can say that this was decisive, or this is what elected Trump, but clearly his behavior during the campaign in terms of what he said during the week or two before the election was unacceptable."
Sanders added, "It is interesting that [Comey] is not doing investigations about the possible, possible tie-in between Trump's campaign and the Russians."
The FBI Director in a closed-door intelligence briefing for members of Congress refused to confirm or deny whether his agency is investigating possible contacts or connections between the Trump campaign and Russia, sources have told ABC News.
But incoming White House chief of staff Reince Priebus said on "This Week" that the president-elect has confidence in the FBI director. "We've had a great relationship with him over the last several weeks ... His term extends for some time yet. There's no plan at this moment in changing that term."
Stephanopoulos also asked Sanders if he thinks Trump will be a "legitimate president," after Democratic Rep. John Lewis and Sen. Cory Booker both said the incoming president will lack legitimacy,
"He's going to be inaugurated this week,” Sanders said. "I have great concerns, and apparently Republicans do as well, and there's going to be an investigation about the role that Russian hacking played in getting him elected."
Rep. Lewis told NBC News on Friday that he does not believe Trump is a legitimate president, and the president-elect slammed the civil rights icon on Twitter the next morning, sparking an intense backlash.
Among those taking Trump to task was New Jersey's Sen. Booker who tweeted “Anyone who attacks @repjohnlewis loses legitimacy in my eyes, especially someone who made such a craven effort to delegitmize @BarackObama”
Sanders told Stephanopoulos, “What Cory Booker and John Lewis are right about is to talk about the racist past of Donald Trump. We all remember that Trump was one of the leaders of the so-called birther movement trying to delegitimize the presidency of our first African-American president, Barack Obama, which is an outrage.”
Unlike Lewis and some other Democrats who have said they will skip Trump's inauguration Friday, the Vermont senator said he plans to attend.
Sanders also warned Republicans about repealing Obamacare without anything to take its place.
"The vast majority of the American people agree with me and many others. You don't simply repeal the Affordable Care Act without a replacement," he said.
Sanders and other Democratic leaders are holding rallies across the country Sunday to organize grassroots support for Obamacare and Social Security, keep the political pressure on Republicans and try to stop the repeal.
Still, Sanders left the door open to working across the aisle to improve the health care law.
“Nobody thinks that Obamacare is perfect. It has its problems,” he said, but, “Sensible people have got to work together.”