WASHINGTON — 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.

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 to authorize the war in Iraq.

Clinton called that a “low blow” and pressed she’s the candidate with the ability to implement progressive changes.