(CNN) Sen. Elizabeth Warren is laying down a new rule for her presidential campaign: No fundraisers, dinners, receptions or phone calls with wealthy donors.

Warren's bid for the White House has been defined since its start by themes of fighting corruption and money in politics. The Massachusetts Democrat took that to the next level on Monday, blasting out an email to supporters vowing to forgo any "fancy receptions or big money fundraisers only with people who can write big checks," as well as phone calls to wealthy donors.

"For every time you see a presidential candidate talking with voters at a town hall, rally, or local diner, those same candidates are spending three or four or five times as long with wealthy donors -- on the phone, or in conference rooms at hedge fund offices, or at fancy receptions and intimate dinners -- all behind closed doors," Warren wrote. "When I thank the people giving to my campaign, it will not be based on the size of their donation."

Warren's announcement Monday is an implicit challenge to -- and perhaps even criticism of -- some of her competitors in the Democratic field who have courted big-dollar donors and bundlers.

The Warren campaign has not held a single fundraiser since New Year's Eve when Warren launched her presidential exploratory committee, Warren aides confirmed to CNN.

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