Hillary Clinton and Bernie Sanders are preparing to go head-to-head for the first time, as they vie to be the Democratic presidential pick.
The TV debate in New Hampshire is their first since the Democratic race was whittled down to two this week.
The first state-by-state contest, in Iowa on Monday, was won narrowly by Mrs Clinton ahead of Mr Sanders.
Since that vote, they have been exchanging barbs over who boasts the stronger liberal credentials.
Their arguments centre on who can best deliver on policies dear to the left-wing of the party, such as providing universal healthcare access, improving income equality and protecting worker rights.
At a town hall event on Wednesday in which they appeared on stage separately, Vermont Senator Mr Sanders accused his rival of being a part-time liberal.
Analysis – Anthony Zurcher, BBC News, New Hampshire
Just days before the New Hampshire primary, Hillary Clinton finds herself in a new role – that of the underdog. She’s trailed Bernie Sanders, senator from neighbouring Vermont, by double-digits in the state’s polls for more than a month.
Now, thanks to former Maryland Governor Martin O’Malley’s withdrawal from the Democratic race for president, she’s alone on the debate stage with the upstart rival who came fractions of a percentage point from beating her earlier this week in the Iowa caucuses.
Eight years ago, Hillary Clinton was reeling after a sound beating in Iowa at the hands of Barack Obama, and a surprise win in New Hampshire gave her new life – at least temporarily. It was the start of what would be a drawn-out battle for the nomination that lasted for months.
Although Mrs Clinton’s nationwide standing is stronger than it was in 2008, a defeat in New Hampshire could mark the start of another long nomination fight.
Consequently the former secretary of state is committing time and resources in New Hampshire to stave off such an embarrassment. Will the Granite State give her yet another boost? A strong performance in tonight’s debate could be her best chance for a last-minute reversal of fortune.
He pointed to her vote as a senator to authorise the war in Iraq and the money her campaign receives from Wall Street as evidence.
But she hit back by saying she was a progressive politician who delivered results, and argued she had been fighting liberal causes for decades.
“Good ideas on paper are important, but you’ve got to be able to translate them into action,” she declared.
The debate will be their first without the presence of the former governor of Maryland, Martin O’Malley, who quit the race on Monday night.
And it is the last before Tuesday’s New Hampshire primary vote, the second state to make its choice.
Mr Sanders holds a big lead in polls in the state – one on Thursday put him 20 percentage points ahead.
Both Republican and Democratic parties will formally name their presidential candidates at conventions in July.
Americans will finally go to the polls to choose the new occupant of the White House in November.
More on the Democratic race
- No jubilation in Hillary camp The mood the morning after a narrow win in Iowa
- The socialist candidate for president How the Vermont senator set the race alight
- The Hillary story From first lady to senator to presidential hopeful
- Why are Americans so angry? The underlying forces that explain rise of Trump, Cruz and Sanders
- How a US election works The difference between a primary and caucus, and other questions answered
- Special report: The BBC’s full coverage of the race to the White House