Elizabeth Warren is calling for big structural change. Bernie Sanders wants a political revolution.
They both draw huge crowds and have raised big money on small dollar donations. And they are the only candidates who are all in on Medicare for all.
Now, progressive Democrats have a decision to make between the two candidates.
"The two of them have really been battling for that progressive vote," said Emerson College professor Spencer Kimball.
What they haven't been battling is each other, thanks to an unspoken "nonaggression pact."
"They're trying to keep it as positive as possible," Kimball said. "But at some point, the rubber is going to hit the road."
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But it's anyone's guess when that time will come.
NBC10 Boston asked Sanders a few months ago when he would start drawing a stronger distinction with Warren.
"You know, I'm going to let Sen. Warren, who is a friend of mine, run her campaign, and we are going to run our campaign," he replied.
Warren was asked a similar question.
"You know, Bernie and I have been friends for many, many years, long before I ever got in politics," she said. "And I don't see any reason that that should change."
Many feel it will have to change — soon — if one of the two is ever able to have a breakout moment.
One school of thought is in Sanders' favor.
"If you like socialist programs in the Democratic platform, you might as well actually vote for the socialist," argued Democratic political consultant Scott Ferson.
Others feel Warren's embrace of capitalism make her a better candidate in the general election.
Still, Sanders isn't going there.
"I don't want to talk about — if it's OK with you, Elizabeth will defend herself," he said over the summer.
Progressive Democrats have been waiting decades for a candidate who could give them an actual shot at the White House. Now, they hope having two viable candidates doesn't somehow ruin that opportunity.