Former U.S. Vice President Joe Biden was at the center of attention Thursday on the second night of Democratic presidential debates, set to make the case that he has the best chance to oust Republican President Donald Trump in the 2020 election.

Biden currently leads Democratic voter preference surveys for the party’s presidential nomination, but he will face some of his biggest rivals before a live audience in Miami, Fla., with millions watching on national television.

He was to be joined in the debate by nine other presidential aspirants, including Vermont Sen. Bernie Sanders, a self-described democratic socialist; California Sen. Kamala Harris, a former state attorney general; and Mayor Pete Buttigieg of the Midwestern city of South Bend, Ind.

FILE – Former Vice President Joe Biden arrives on stage at the South Carolina Democratic Party convention, June 22, 2019, in Columbia, S.C.

Biden, President Barack Obama’s two-term running mate as vice president, twice has failed to win the party’s presidential nomination, in 1988 and 2008. But he has consistently led national polling this year, both over his Democratic rivals for the party nomination and over Trump in a hypothetical general election matchup on Nov. 3, 2020.

Biden’s closest Democratic challengers are Sanders and Sen. Elizabeth Warren of Massachusetts, the key contender among 10 on the debate stage Wednesday, when more than 15 million people tuned in to see the first major political event of the 2020 campaign.

Biden has attempted to portray himself as a steady alternative to the unpredictable Trump, one who would restore frayed U.S. relations with foreign allies and undo conservative domestic policies Trump has adopted.

Democratic presidential candidate Sen. Bernie Sanders pauses while speaking during a forum in Miama, Florida, June 21, 2019.

But more progressive Democrats have questioned Biden’s bona fides and political history over four decades in Washington as the party’s key current figures have aggressively moved toward more liberal stances on a host of key policy issues, including health care, abortion rights, taxes and immigration.

Some critics also have suggested that Biden might be too old to become the U.S. leader. Now 76, Biden would be 78 and the oldest first-term president if he were to defeat the 73-year-old Trump and take office in January 2021. Trump often mocks him as “Sleepy Joe.”

In the Midwestern farm state of Iowa recently, Trump assessed his possible Democratic opponents, saying of Biden, “I think he’s the weakest mentally, and I think Joe is weak mentally. The others have much more energy.”

Biden, for his part, labeled Trump “an existential threat” to the U.S.

Democratic presidential hopeful Massachusetts Senator Elizabeth Warren participates in the first Democratic primary debate of the 2020 presidential campaign at the Adrienne Arsht Center for the Performing Arts in Miami, June 26, 2019.

On Wednesday, Warren, a progressive lawmaker and one-time Harvard law professor, declared, “I want to return government to the people.”

Referring to major corporations, she added, “What’s been missing is courage, courage in Washington to take on the giants. I have the courage to go after them.”

Later, Warren said she supports a government-run health care system, as does Sanders, that could end the private insurance-based health care now used in the U.S. Some Democratic candidates and most Republicans, including Trump, oppose such a change as costly and a mistake for the country.

But Warren said, “Health care is a basic human right and I will fight for basic human rights.”

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