Hillary Clinton and Bernie Sanders are holding their first one-on-one debate, 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.
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 is 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.