Former president Barack Obama said the Republican Party's failure to reject his successor Donald Trump's false claim that he won the 2020 election poses a threat to US democracy.

Obama, in an interview with CNN television broadcast on Monday evening, said Republican lawmakers have been "cowed" into accepting Trump's lie that he defeated Democrat Joe Biden.

This culminated in the January 6 attack on the US Capitol by a mob of Trump supporters who were seeking to block certification of Biden's victory in the presidential election, he said.

"Originally it was 'Oh, don't worry this isn't going anywhere' and 'We're just letting Trump and others vent,'" Obama told CNN's Anderson Cooper.

"And then, suddenly, you now have large portions of an elected Congress going along with the falsehood that there were problems with the election," he said.

"And nobody stood up and said 'Stop, this is enough, this is not true,'" Obama said.

"I won't say nobody, let me correct that, there were some very brave people who did their jobs," he said, referring to election officials.

But, Obama said, "all those (Republican) congressmen started looking around and they said 'You know what I'll lose my job'" if I don't back Trump's claims.

Trump has repeatedly attacked Republican members of Congress such as Wyoming Representative Liz Cheney who have rejected his false assertion that he won the election.

"I didn't expect there would be so few people who would say 'Well, I don't mind losing my office cause this is too important, America's too important, our democracy's too important,'" Obama said. "We didn't see that."

"We have to worry when one of our major political parties is willing to embrace a way of thinking about our democracy that would be unrecognizable and unacceptable even five years ago or a decade ago," he said.

Obama, America's first Black president, said he remained optimistic and referred to his 2008 election slogan.

"I'm still the hope and change guy," he said. "And so my hope is that the tides will turn.

"But that does require each of us to understand that this experiment in democracy is not self-executing," he said. "It doesn't happen just automatically."