Twelve Democratic White House hopefuls are squaring off for a presidential debate that could be a critical test for the front-runners.
Joe Biden is under siege from personal attacks by President Donald Trump, and Bernie Sanders is recovering from a heart attack.
Elizabeth Warren is likely to face new scrutiny after surging in the polls.
The Democratic White House race officially begins with the Iowa caucuses on 3 February.
The pack will be whittled down until a White House nominee is crowned at the party convention next July before he or she faces Mr Trump, a Republican, in the November 2020 election.
Tuesday night’s debate, hosted by CNN and the New York Times, in the electoral battleground state of Ohio will be the most crowded so far in the Democratic race.
Also on stage are South Bend, Indiana, mayor Pete Buttigieg, California Senator Kamala Harris, New York entrepreneur Andrew Yang, former Texas congressman Beto O’Rourke, Minnesota Senator Amy Klobuchar, New Jersey Senator Cory Booker, former Obama housing secretary Julian Castro and Hawaii congresswoman Tulsi Gabbard and billionaire activist Tom Steyer.
Languishing on single digits in the opinion polls, they will be striving to make a splash with time running out.
It is the first debate since congressional Democrats launched an impeachment inquiry into Mr Trump’s efforts to pressure Ukraine to investigate Joe Biden.
Mr Biden is likely to be asked about Mr Trump’s unsubstantiated claims that the former US vice-president improperly tried to aid his son Hunter Biden’s business interests in Ukraine.
His Democratic rivals have so far refrained from attacking Mr Biden on the issue.
But Mr Biden will be aiming to steady his campaign after seeing his once commanding lead in opinion polls erode.
Mr Sanders, a Vermont senator, will be closely watched in the three-hour debate for signs of flagging stamina after suffering a heart attack earlier this month.
The oldest contender at 78, Mr Sanders has dropped into third place in the polls.
Ms Warren, a Massachusetts senator, may find a bullseye on her back on Tuesday night as she has accelerated to the tip of the field in the past two months.
Mr Biden recently jabbed at her “I have a plan for that” policy catchline, telling a New Hampshire crowd last week: “We’re not electing a planner.”
Mr Sanders, a progressive ally of Ms Warren, also took a swipe at her this past weekend.
“Elizabeth I think, as you know, has said that she is a capitalist through her bones. I’m not,” self-described democratic socialist Mr Sanders told ABC on Sunday.
Both Mr Sanders and Ms Warren favour an NHS-style free healthcare for Americans.
But unlike Mr Sanders, Ms Warren has repeatedly avoided saying whether her proposal would raise taxes on working Americans families.
Ms Warren has recently been under attack from Republicans about whether she was actually forced from her teaching job because of pregnancy half a century ago.
Critics have claimed there are inconsistencies in her story.
Ms Warren has previously apologised for a widely ridiculed claim of Native American heritage.
Who will take on Trump in 2020?
Election day is still more than a year away but the race to become the Democratic challenger to Mr Trump is already well under way.
Joe Biden and Bernie Sanders have thrown their hats into the ring, but most of the other candidates are relatively unknown outside the Washington DC bubble.