Back in Iowa as a full-fledged presidential candidate, Democrat Elizabeth Warren took aim at President Donald Trump on Sunday, saying he "may not even be a free person" by next year's election.

The Massachusetts senator's comments came a day after Trump renewed his criticism of her past claims of Native American heritage. In a tweet, Trump called Warren "Pocahontas" and said he would see her "on the campaign TRAIL."

The White House didn't explain what the president was referring to in his tweet, though some Democrats accused him of making light of the Trail of Tears — the forced removal of Cherokee and several other Native American tribes from their lands in the 1830s. Warren's campaign wouldn't say what the senator believes Trump was referencing.

Warren has largely avoided talking about Trump since she began testing the waters for a campaign more than a month ago. During her first of three events Sunday in eastern Iowa, Warren said the president shouldn't be allowed to dictate the direction of the campaign with divisive attacks.

"Every day there is a racist tweet, a hateful tweet — something really dark and ugly," she said. "What are we as candidates, as activists, as the press, going to do about it? We're going to chase after those every day?"

She continued: "Here's what bothers me. By the time we get to 2020, Donald Trump may not even be president. In fact, he may not even be a free person."

Warren didn't explain her comment, though she appeared to be referring to the multiple investigations that have shadowed Trump's presidency. Asked later if she supported impeaching Trump, Warren was noncommittal, saying only that special counsel Robert Mueller needs to be allowed to finish his investigation and make his report public.

"If we go down that path, we're going to need to help pull this country together and have as many people as possible understand it was a legitimate process based on facts," she said.

Trump has not been charged with any crimes, but several of his former advisers have been pleaded guilty to a variety of charges.

Warren formally launched her White House bid Saturday in Massachusetts, grounding her campaign in the populist calls to combat economic inequality that have long made her a favorite of liberals. In Cedar Rapids, she said sweeping changes are needed to rebalance an economy that she now favors the wealthy at the expense of middle- and lower-income Americans.

Warren has struggled to shake the controversy surrounding her past claims to Native American heritage. She's apologized for claiming Native American identity on multiple occasions early in her career. Trump has frequently taken digs at the senator by calling her Pocahontas, a reference to the native woman who lived in present-day Virginia in the 1600s and agreed to marry an English colonist to help ensure peace and protect her people.

Warren has said the story of Pocahontas long has "been taken away by powerful people who twisted it to serve their own purposes."

Voters in Cedar Rapids did not ask Warren about the controversy during a question-and-answer session, focusing instead on her stances on issues including tariffs and student loan debt.