US President Donald Trump Friday said the opposition Democrats want to impeach him because they know that they cannot win the 2020 presidential election.

On Thursday, the Democrats became the majority party in the 435-member US House of Representatives. This has ended the monopoly the ruling Republicans had enjoyed for the last two years in Congress and the White House.

"They (Democrats) only want to impeach me because they know they can't win in 2020, too much success!" Trump said in a series of tweets.

"As I have stated many times, if the Democrats take over the House or Senate, there will be disruption to the financial markets. We won the Senate, they won the House. Things will settle down," Trump said.

Trump questioned the very idea of impeaching him, claiming he has been a very successful president.

"How do you impeach a president who has won perhaps the greatest election of all time, done nothing wrong (no collusion with Russia, it was the Dems that colluded), had the most successful first two years of any president, and is the most popular Republican in party history 93 per cent (sic)?" Trump wrote.