Democrats in the U.S. House of Representatives on Tuesday launched a formal impeachment inquiry into President Donald Trump, accusing him of seeking foreign help to smear Democratic rival Joe Biden ahead of next year's election.

The inquiry ensures a partisan fight in Congress and on the presidential campaign trail in coming months, energizing the most committed Republican and Democratic supporters and overshadowing the Democratic race for the nomination to face Trump in the November 2020 election.

House Speaker Nancy Pelosi announced the inquiry after a closed-door meeting with Democratic lawmakers, saying Trump's actions undermined national security and violated the U.S. Constitution.