Vermont senator Bernie Sanders has called on Donald Trump to apologise for his inflammatory rhetoric during the presidential campaign.
He has also asked him cut ties with campaign chief executive Steve Bannon, named as a top White House adviser on Monday by the president-elect.
That sparked an outcry from Democrats, who blasted the conservative media chief executive as peddling conspiracy theories and white supremacy.
Mr Sanders, who lost the Democratic presidential nomination to Hillary Clinton, says his office received "many, many" calls asking Mr Trump to withdraw the appointment.
"What we are seeing all over this country is extraordinary fear," Mr Sanders said at a breakfast sponsored by the Christian Science Monitor.
He said Mr Trump should try to "make the American people feel comfortable" by rescinding Mr Bannon's nomination.
The self-identified independent also said there may be opportunities for Democrats to work with Trump on some of his campaign promises including curbing high pharmaceutical drug pieces, renegotiating trade deals, cutting the influence of Wall Street and other ideas that will "improve life for working people".
"He said a whole lot of things. Was he serious or were these just campaign slogans?" he said. "We will hold him accountable."
Mr Sanders told reporters that he has not spoken personally to Mrs Clinton but believes she has an important role to play in the party's future.
She made her first public remarks since her concession speech in Washington on Wednesday evening, urging her supporters to "stay engaged" in political activism.
Democrats, he said, must do some "soul searching" in the coming months.
Mr Sanders is backing Minnesota representative Keith Ellison to head up the Democratic National Committee, arguing the first Muslim-American member of Congress could bring some fresh ideas to the party.
"It is time for the Democratic party to reassess what it stands for and where it wants to go," he said.