Harvard Law School professor Laurence Tribe on Monday took a swipe at 2020 presidential candidate Sen. Bernie Sanders Bernie SandersOutrage erupts over Breonna Taylor grand jury ruling Dimon: Wealth tax 'almost impossible to do' Grand jury charges no officers in Breonna Taylor death MORE (I-Vt.), calling him a “phony.”

Tribe tweeted that he was making his assertion “based on watching [Sanders] for decades,” while also saying he did not think the Independent senator is “a monster” like he believes President Trump Donald John TrumpBiden on Trump's refusal to commit to peaceful transfer of power: 'What country are we in?' Romney: 'Unthinkable and unacceptable' to not commit to peaceful transition of power Two Louisville police officers shot amid Breonna Taylor grand jury protests MORE to be.

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“I’d prefer him massively over Trump. But I’d prefer a cardboard box over Trump,” Tribe added.

How can I put it more simply? Based on watching him for decades, I believe @BernieSanders is a phony. Not a monster like @realDonaldTrump but a phony. I’d prefer him massively over Trump. But I’d prefer a cardboard box over Trump. — Laurence Tribe (@tribelaw) May 6, 2019

Tribe did not give details about why he is accusing Sanders of being a "phony." He did not immediately reply to a request for comment from The Hill.

The law professor, who has been highly critical of Trump, faced some criticism from leftist voices over the remarks.

For example, The Intercept reporter Lee Fang resurfaced a report that Tribe was hired by Peabody Energy Corp., the nation’s biggest coal company, to oppose the Environmental Protection Agency’s Clean Power Plan.

Real, non-phonies like Prof. Tribe are paid $75,000 per month by Peabody Coal Company to help overturn climate regulations on coal-burning power plants https://t.co/NOLQg3lsMp — Lee Fang (@lhfang) May 6, 2019

Sanders, a self-described democratic socialist, is seen as a top 2020 Democratic contender, but has taken flak for his progressive views, while others have taken shots at him for his more conservative voting record on some issues like gun rights.

Some establishment Democrats also feel lingering wounds from the 2016 Democratic primary, when Sanders refused to concede against former Secretary of State Hillary Clinton Hillary Diane Rodham ClintonDemocratic groups using Bloomberg money to launch M in Spanish language ads in Florida The Hill's Campaign Report: Presidential polls tighten weeks out from Election Day More than 50 Latino faith leaders endorse Biden MORE until late in the process.

Sanders's decision to remain an Independent has also stirred debate, being criticized privately by some Democrats. But the decision has also earned him praise from some progressives who say they don't trust party leadership after the 2016 battle.