Former U.S. Vice President Joe Biden entered the 2020 Democratic presidential field on Thursday as an instant front-runner, drawing momentum away from other leading candidates and putting new pressure on underperformers to find ways to stay relevant.

Biden, 76, a longtime U.S. senator who served two terms as then-President Barack Obama's No. 2, announced his bid in a video describing the high stakes of the race to take on Republican President Donald Trump in next year's election.

"We are in the battle for the soul of this nation," Biden said. "I believe history will look back on four years of this president and all he embraces as an aberrant moment in time. But if we give Donald Trump eight years in the White House, he will forever and fundamentally alter the character of this nation, who we are, and I cannot stand by and let that happen."