Josh Hafner

USA TODAY

Some onlookers have framed the Democratic nomination race as a clash between Hillary Clinton's pragmatism and Bernie Sanders' idealism — a struggle of head versus heart.

It's a dichotomy Sanders and Clinton have taken to, as well.

On Wednesday, the campaigns' differences boiled over onto Twitter in a debate over which candidate is truly "progressive."

It started on Tuesday, when MSNBC asked Sanders whether he thought Clinton was a progressive.

"Some days, yes," Sanders said, "except when she announces that she is a proud moderate. And then I guess she's not a progressive."

On Wednesday, Clinton called Sanders' comment a "low blow," touting her past accomplishment. “If it’s about our records, hey, I’m gonna win by a landslide,” Clinton said, per MSNBC.

Later that day, Sanders continued his line of argument from the day before in a series of statements on Twitter aimed at Clinton:

"You can be a moderate. You can be a progressive. But you cannot be a moderate and a progressive." "Most progressives that I know don't raise millions of dollars from Wall Street." "Most progressives I know are firm from day 1 in opposition to the Trans-Pacific Partnership. They didn't have to think about it a whole lot." "Most progressives that I know were opposed to the Keystone pipeline from day one. Honestly, it wasn’t that complicated." "Most progressives I know were against the war in Iraq. One of the worst foreign policy blunders in the history of the United States."

Later, that afternoon, Clinton's campaign fired back:

A little more than an hour later, Sanders mimicked his rival's response:

Expect this debate to move Twitter to the television, as Sanders and Clinton take stage for Thursday's New Hampshire Democratic debate.

How to watch the New Hampshire Democratic debate