Washington: Democratic frontrunner Joe Biden has openly called for Congress to impeach President Donald Trump, accusing him of undermining US democracy and obstructing justice.

"Donald Trump has violated his oath of office, betrayed this nation, and committed impeachable acts," Biden told supporters at a campaign stop in New Hampshire on Wednesday.

"To preserve our constitution, our democracy, our basic integrity, he should be impeached."

Trump is facing an impeachment inquiry in the Democratic-controlled House of Representatives over his contacts with Ukraine, including asking that country to investigate Biden and his son, Hunter Biden, in a move seen as soliciting foreign help in his re-election campaign.