Sen. Elizabeth Warren, D-Mass., thinks billionaires shouldn't get to use their money to overpower the will of the grassroots.

"I think this is a moment for all of the Democratic nominees as they come into the race to say, in a Democratic primary, we are going to link arms, and we're going to say, grassroots funding. No to the billionaires. No to the billionaires, whether they are self-funding or whether they're funding PACs," Warren told MSNBC, referring to political action committees.

When asked whether that precluded contenders like former New York City Mayor Michael Bloomberg and hedge fund manager Tom Steyer from entering the fray for the 2020 Democratic presidential nomination, Warren said, "Of course not." Both Bloomberg and Steyer are openly considering bids for the White House.

"I just mean that people should not be self-funding, and they should not be funded from PACs from other billionaires," she said. "A primary is an opportunity to hear from the grassroots, to see what you can build, to see what kind of energy is out there. Get out there, trust your message, trust what it is that you're fighting for. And if someone else wants to fight for something different, trust them to get out there and fight it. And then let's see where grassroots America is."

The Massachusetts Democrat, a Wall Street critic who helped establish the Consumer Financial Protection Bureau, said President Trump is an example of "what happens when corruption invades a system."

"Donald Trump is an accelerant," she said. "He takes a problem that has been growing and growing and growing and he just sets it off and makes it worse than ever."

Warren this week joined former Obama administration Housing and Urban Development Secretary Julian Castro in forming a presidential exploratory committee. This weekend she is expected to tour Iowa, an early voting state, as she continues to mull whether she will officially run for the White House.