Hillary Clinton and Bernie Sanders are in a tug-of-war over who's the best standard-bearer for progressive values as they road test lines of argument for the first one-on-one debate of the Democratic campaign.

The race for the Democratic nomination, once seen as a sure thing for Clinton, took on new vigor this week after Sanders held the former secretary of state to a whisper-thin margin of victory in Iowa's leadoff caucuses. The tone of their back-and-forth has become increasingly sharp this week, and the candidates agreed to add four more debates to the primary season schedule, including Thursday's faceoff in Durham, New Hampshire.

In a tussle over their very political identities, the two candidates are engaged in an ongoing argument over who is more committed to — and capable of — carrying out a liberal agenda on health care, income inequality, worker rights and more.

Sanders, favored in New Hampshire's first-in-the-nation primary, said Wednesday that Clinton's record is "just not progressive" on any number of issues, including her vote as a senator to authorize the war in Iraq.

Clinton called that a "low blow" and pressed her counterpoint that she's the candidate with the ability to actually implement progressive changes.

"Good ideas on paper are important, but you've got to be able to translate them into action," she said.

The two made their rival cases in interviews and appearances around New Hampshire and in back-to-back appearances at a town-hall style forum on CNN on Wednesday night.

Polls find Sanders holding a commanding lead over Clinton in New Hampshire, and he was eager to lower expectations for how he would finish. He cast himself as an underdog going up against "the most powerful political organization in the country."

Clinton, for her part, has signaled her determination at least to narrow the gap before Tuesday's vote. And her prospects are much stronger after Iowa, as the race moves on to states with more diverse electorates that are to her advantage.

The two campaigns even skirmished over why Sanders was doing so well in New Hampshire polls. Sanders' campaign accused Clinton's of insulting New Hampshire voters by suggesting that they only support the Vermont senator because he's from a neighboring state. That was after Clinton's campaign manager referred to New Hampshire as Sanders' "backyard."

On the broader issue, Clinton offers herself to voters as "a progressive who gets things done," part of her pitch that she's the one with the practical skills to implement a progressive agenda.

Sanders counter-argument is that it will take a "political revolution" to achieve goals such as universal health care, a fairer tax system and an incorruptible campaign finance system.

Asked earlier in the week if Clinton was a progressive, he said: "Some days, yes. Except when she announces that she is a proud moderate. Then I guess she is not a progressive."

Sanders added that it was hard to take on the establishment "when you become as dependent as she has through her super PAC and in other ways on Wall Street or drug company money."

Clinton said she found it amusing that the senator thinks he can be the "gatekeeper" of who's progressive.

The Durham debate will be the first faceoff for Clinton and Sanders since former Maryland Gov. Martin O'Malley dropped out of the race after a poor showing in Iowa.

The candidates next meet on Feb. 11, then on March 9 for a debate that has long been on the schedule. Under an agreement announced Wednesday, there will also be a March debate in Flint, Michigan, where the city's water contamination crisis has been attracting national attention, and two debates in April and May on dates still to be determined.

