(CNN) Massachusetts Sen. Elizabeth Warren listed her race as "American Indian" on a State Bar of Texas registration card in 1986, the Washington Post reported Tuesday.

The Post's reporting represents a previously unknown instance of Warren claiming her race as Native American and the first document that appears to definitively show Warren making the claim in her own handwriting. According to the Post, Warren has previously declined to answer whether she or an assistant filled out forms in which her race was listed as Native American.

Warren's longstanding assertions of having Native American heritage have dogged her political career and provided fodder for attacks from President Donald Trump. Her use of DNA testing to confirm her limited Native roots last year was met with fierce criticism from some Native American groups. The prospective 2020 Democratic presidential candidate is expected to formally announce her entrance into the race on Saturday after forming an exploratory committee on New Year's Eve.

The card, obtained by the Post in an open records request, lists Warren's alma maters of the University of Houston and Rutgers Law School but no other racial identifiers.

A Warren aide did not dispute to CNN the card's legitimacy or Warren's handwriting, and noted that the form was not a part of the application to the bar, but rather a registration card to the Texas Bar after the senator was already admitted. The aide declined to comment on whether there might be other unreported examples of Warren having claimed American Indian heritage.

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