WASHINGTON (Reuters) – Former U.S. Vice President Joe Biden, a moderate who has made his appeal to working-class voters who deserted the Democrats in 2016 a key part of his political identity, launched a bid for the White House on Thursday as the party’s instant frontrunner.

Biden announced the third presidential bid of his career by video on YouTube and other social media. He is expected to make his first public appearance as a candidate on Monday at an event in Pittsburgh featuring union members, a key constituency.

In his video, Biden drew a stark contrast between himself and President Donald Trump.

“Everything that has made America America is at stake,” he said, adding: “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.”

Biden, 76, had been wrestling for months over whether to run. His candidacy will face numerous questions, including whether he is too old and too centrist for a Democratic Party yearning for fresh faces and increasingly propelled by its more vocal liberal wing.

Biden starts as leader of the pack in opinion polls of a Democratic field of 20 contenders seeking a chance to challenge Trump, the likely Republican nominee, in November 2020.

Critics say his standing in polls is largely a function of name recognition for the former U.S. senator from Delaware, whose more than four decades in public service includes eight years as President Barack Obama’s No. 2 in the White House.

Obama’s spokeswoman Katie Hill said in a statement after Biden’s announcement that Obama has long said selecting Biden to be his running mate in 2008 was one of the best decisions he ever made. The statement fell short of a formal endorsement, but said Obama relied on Biden’s insight, knowledge and judgement through both election campaigns and his entire presidency.