HOUSTON, Texas (AFP) — Frontrunner Joe Biden goes head-to-head for the first time Thursday with Elizabeth Warren, his fast-rising rival for the Democratic Party’s White House nomination, in a three-hour debate seen as a tough test of the 76-year-old’s stamina.

Ten candidates will take the stage in Houston as a narrowed 2020 field, including Bernie Sanders, the leftist jockeying for second spot with fellow Senator Warren in the race to challenge President Donald Trump in 2020.

While the top three Democrats are in their seventies, all eyes will be on former vice president Biden who is battling accusations that he is a gaffe-prone candidate past his prime, who could struggle during a gruelling campaign.

The previous debates, in June and July, featured a dizzying 20 hopefuls over two long nights, with months still to go before the first votes are cast in Iowa in February to pick the nominee.

While most of them remain in contention, the lineup has been tightened this time around for the televised showdown airing on ABC from 7 p.m. (0000 GMT).

Bringing America’s diversity to the fore, the 10 debate participants are white, black, Hispanic, and Asian-American; seven men and three women; three septuagenarians and four candidates 30 to 40 years their junior; centrists, progressives and far-left liberals.

Addressing reporters shortly before the debate, Trump predicted that barring a “major mistake,” Biden would eventually clinch the Democratic nomination.

“It’s going to be one of those three,” Trump said. “I would imagine Biden would be able to make it if he doesn’t make any major mistakes.”

Trump has routinely mocked Biden — branding him “Sleepy Joe” and suggesting he has lost a step — as well as Warren and Sanders, but he struck a quite different tone when asked if he respected any of his would-be challengers.