(CNN) For months, Elizabeth Warren has struggled to get from under the muddled -- and occasionally contradictory -- story she has told about her own belief in her Native American heritage, and whether she ever either claimed that background on documents or received any sort of advantage from it.

On Tuesday, Warren's claims took a major hit: The Washington Post reported on a Texas bar registration card from 1986 in which, clearly and apparently in her own hand, the Massachusetts Democrat had written "American Indian" when asked to identify her race.

Neither Warren nor her office denied either the authenticity of the document nor that it was the senator's signature on it. "I can't go back," Warren told the Post in an interview Tuesday. "But I am sorry for furthering confusion on tribal sovereignty and tribal citizenship and harm that resulted."

Here's why this all matters so much: Prior to the Post's report of this bar registration card, Warren had maintained some level of plausible deniability about claiming herself as a Native American. As the Post wrote:

"The Texas bar registration card is significant, among other reasons, because it removes any doubt that Warren directly claimed the identity. In other instances Warren has declined to say whether she or an assistant filled out forms."

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