Sen. Bernie Sanders, I-Vt., refused to say Thursday if he struck a secret deal with his fellow 2020 presidential candidate Sen. Elizabeth Warren, D-Mass., not to attack each other on the debate stage.

During an interview on CBS, anchor Anthony Mason asked point-blank if there was a private pact between the two Progressives and Sanders sidestepped the question without giving a yes or no answer.

"You appeared, in the debate the other night ... to have kind of a nonaggression pact with Elizabeth Warren. Do you two have a deal not to attack each other -- at least at this stage?" he was asked.

"Look, Elizabeth and I have been friends for over 20 years. She's running her campaign, and I'm running my campaign," Sanders replied. "They're different campaigns."

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Sanders then pivoted to the issues of middle-class economics and climate change, before vaguely saying that it would be a poor strategy to attack other candidates.

"I think the most effective way to campaign, to be honest with you, is to talk to the American people about why the middle class is disappearing," he said. "The great challenge we face in terms of climate change, the thievery that’s now going on within the pharmaceutical industry, where there is not only price-fixing but just incredible greed. You talk about those issues, you do well. You try to beat up on somebody else, frankly, I don't think it's good politics."

"CBS This Morning" co-anchor Tony Dokoupil said it falls on Sanders to make a distinction between himself and Warren after the Vermont lawmaker said he would be leaving that decision up to the voters and pundits.

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"No [I don't have to differentiate]. I have to tell the Americans what I believe. And I'm going to take on the greed of Wall Street, the drug companies, [and] the insurance companies," he replied.

"We are going to tell the fossil fuel industry that they cannot continue to destroy this planet for their short-term profits. Right now the American people understand there’s something wrong when many people are working longer hours for lower wages. Forty percent of all new income goes to the top one percent. That’s my message to the American people. We’ve got to take on corporate greed."