WASHINGTON – Presidential hopeful Sen. Elizabeth Warren is facing further fallout from her past self-identification as Native American after the Washington Post on Tuesday published a copy of her 1986 State Bar of Texas registration card on which she listed her race as "American Indian."

The Post said it obtained the document, which was filled out by hand and signed, through an open records request, and that the Massachusetts Democrat's office did not question its authenticity. It is dated April 18, 1986.

According to the Post, it is the first record from Warren's past where she made her claim of Native American heritage in her own handwriting, which proves she was directly responsible for the dubious identification. Warren had previously declined to answer if she or an assistant had filled out the forms where she made the claim.

"I can’t go back," Warren told the Post. "But I am sorry for furthering confusion on tribal sovereignty and tribal citizenship and harm that resulted."

Last week, Warren met privately with the chief of the Cherokee Nation to apologize for identifying herself as Native American. She reportedly met primarily to express her regret for her October release of a DNA test she tried to use to establish the legitimacy of her claim.

The Post asked her if her apology was meant to include identifying herself as Native American at the University of Pennsylvania and at Harvard University, as well as marking herself as a minority with the Association of American Law Schools.

"Yes," she said. "I told him I was sorry for furthering confusion about tribal citizenship.

"I am also sorry for not being more mindful about this decades ago," she added.

President Donald Trump has long mocked Warren's claims of Cherokee blood, referring to her as "Pocahontas" in tweets and speeches. That label, and other taunts of Warren rooted in Native American stereotypes, have been criticized as racist.

Trump has said for months that he hopes Warren wins the Democratic nomination in 2020 because he thinks he could hammer her on the race issue.

"We can finally get down to the fact as to whether or not she has Indian blood," he said at a rally in October. "And she's gotten a lot of advantages by falsely claiming what she's claiming."

Trump and other conservatives have accused Warren of using her claim of minority status to benefit her in her academic and professional life.

But after a review of her personnel files from the University of Pennsylvania and Harvard, and more than 100 interviews, The Boston Globe concluded in September that, "It is clear that Warren was viewed as a white woman by the hiring committees at every institution that employed her."

It was not until after she was hired that Warren changed her ethnicity from white to Native American at the University of Pennsylvania Law School, where she worked from 1987 to 1995, the Globe reported.

Similarly, she changed her ethnicity in Harvard's human resources system four months after she began working there as a tenured professor in 1995. The Ivy League institution identified Warren as Native American on federal affirmative action forms every year until 2004, according to the Post.

In 1986, Warren also began registering herself as a minority with the Association of American Law Schools. She continued to do so through 1995, the Post reported.

Warren had hoped that the unveiling of her DNA test would put the matter behind her. But instead the test – which showed she did have American Indian ancestry but that it dated back at least six generations – only drew her more mockery from her critics, while simultaneously outraging representatives of the Cherokee Nation.

The issue could damage Warren's 2020 chances if it is perceived as a liability that could make her a weaker candidate to take on Trump, even among Democrats who like her policies. A recent Monmouth University poll found that 57 percent of registered Democrats and Democratic-leaning independents would rather vote for a candidate they don't agree with, but who can beat Trump, than a candidate they agree with who would have a tough time in the general election.