Sen. Elizabeth Warren wants billionaires like Howard Schultz and Michael Bloomberg to subscribe to the United States' "social contract" and pay "their fair share" in taxes, she told CNBC on Thursday in an interview with Jim Cramer.

"I want these billionaires to stop being freeloaders," the Democratic senator said on "Mad Money." "I want them to pick up their fair share. That's how we make a system that works not just for the rich and the powerful, but works for all of us."

Warren, who is considering a 2020 run for president, proposed a "wealth tax" on Americans with over $50 million in assets earlier this month. She has also criticized former Starbucks CEO Howard Schultz — who is also weighing a 2020 presidential bid — for thinking he can "buy the presidency."

On Thursday, the Massachusetts senator broke down her problem with billionaires, even those who contribute to charities and do good social works.

"We have watched billionaires stand up and say, 'Look, I want to run for president. And one of the first planks in my plan is going to be no new taxes for billionaires,'" she told Cramer.

"The thing about taxes is everybody who is an ultra-millionaire has to pay a portion, not just those who sign up, not just those who wave their hands and say, 'I'll do it as long as it goes to the particular charity I like,'" Warren continued. "That is your obligation. That's part of the social contract. That's part of being a citizen of the United States of America."

A 2 percent tax, for example, was "not unreasonable" to impose on the "thinnest one-tenth of 1 percent" who earn multimillions, Warren said, adding that that portion of the U.S. population pays roughly 3.2 percent of their total worth in taxes, compared with the 7.2 percent the 99 percent pays.

That additional money could go to child care, lowering student loan debt, or a Green New Deal, she added.

"All I'm asking for is a little slice from the tippy, tippy top. A slice that would raise — and this is the shocking part, Jim — about $2.75 trillion over the next 10 years," the senator said. "That's money we need so that every kid in this country has a decent childcare opportunity, has an opportunity for pre-K, has an opportunity for a decent school."