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(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.

“As Senator Warren has said she is not a citizen of any tribe and only tribes determine tribal citizenship. She is sorry that she was not more mindful of this earlier in her career,” said Kristen Orthman, a spokeswoman for Warren, in a statement.

Republican National Committee spokesman Mike Reed accused Warren of apologizing at a time that was politically convenient.

“Now, four days before her formal presidential launch, she’s issued a politically opportunistic apology that doesn’t go nearly far enough,” Reed said in a statement. “Warren pretended to be a minority to climb the Ivy League ladder – a lie that will continue to haunt her presidential ambitions.”

Responding to the accusation that Warren’s ethnicity factored into her rise in the legal world, an aide pointed to an exhaustive Boston Globe investigation published last year. It concluded that there was “clear evidence, in documents and interviews, that her claim to Native American ethnicity was never considered by the Harvard Law faculty, which voted resoundingly to hire her, or by those who hired her to four prior positions at other law schools.” A controversial video that Warren released in October showing the results of her DNA test also featured some of Warren’s former colleagues in academia, who stated that the senator’s heritage did not affect her hiring.

News of the card comes following Warren’s acknowledgment Monday that she had apologized to Cherokee leaders for causing “confusion” by her use of a DNA test to prove Native American ancestry.

“I’m not a tribal citizen and I respect the difference,” the Massachusetts Democrat told CNN in the Capitol on Monday. “Tribes and only tribes determine tribal citizenship.”

Warren said that she told the Cherokees’ Principal Chief Bill John Baker last week that she was “sorry for adding confusion about tribal citizenship and tribal sovereignty and for harm caused to native tribes – and also for not being more mindful of that decades ago,” Warren said.

The Massachusetts Democrat announced last fall that she had taken a DNA test indicating “strong evidence” of Native American ancestry “6-10 generations ago.” But the move incited further criticism given the distant nature of her connection to the tribe.

Cherokee Nation executive director of communications Julie Hubbard said in a statement last week that Warren had apologized to the tribe.

“We are encouraged by this dialogue and understanding that being a Cherokee Nation tribal citizen is rooted in centuries of culture and laws not through DNA tests,” Hubbard said. “We are encouraged by her action and hope that the slurs and mockery of tribal citizens and Indian history and heritage will now come to an end.”

CNN’s MJ Lee, Manu Raju and Gregory Krieg contributed to this report.