Massachusetts Senator Elizabeth Warren has apologized for ever claiming Native American ethnicity, as she competes with a slew of Democrats in the presidential candidate field.

The 69-year-old Democrat responded after the emergence of a State Bar of Texas registration card showing she identified as 'American Indian' on April 18, 1986.

She left the 'National Origin' and 'Physical Handicap if any' parts of the form blank however, possibly supporting the notion of critics like President Donald Trump who have claimed she used the idea of being part of a minority group to advance her career.

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Massachusetts Senator Elizabeth Warren has apologized for the second time in one week for claiming Native American ethnicity. Warren is pictured arriving for the State of the Union address at the US Capitol in Washington, DC, on Tuesday

Donald Trump Jr. tweeted suggesting a handwritten document showing she identified as 'American Indian' was leaked on the same day as the SOTU address so it would go under radar

'I can't go back,' Warren told The Washington Post Tuesday. 'But I am sorry for furthering confusion on tribal sovereignty and tribal citizenship and harm that resulted.'

While other typed up evidence has been presented showing Warren did identify as Native American in various jobs as a law professor, she hadn't confirmed whether she personally used the labels or whether information was filled out on her behalf.

But the unearthing of the yellow card that lists the address of the University of Texas law school in Austin, where she was working at the time, pinpoints that she did.

It's not clear how the labeling benefited Warren if it did.

A small typed note on the card reads: 'The following information is for statistical purposes only and will not be disclosed to any person or organization without the express written consent of the attorney.'

Donald Trump Jr. used the revelation to suggest it was purposely revealed on the same day as the State of the Union in order for it to go under the radar.

'Suspicious timing for this to come out,' he tweeted. 'It’s almost like Elizabeth Warren is trying to hide what I’ve said all along about taking advantage of her fake Native American heritage at the expense of real Native Americans and minorities.'

'I can't go back,' Warren told The Washington Post Tuesday. 'But I am sorry for furthering confusion on tribal sovereignty and tribal citizenship and harm that resulted'

Her race identification for the Bar exam came the same year she was recorded as a minority for the Association of American Law Schools.

She listed herself as such annually until 1995, the year she started working at Harvard Law School.

Several months after starting there she put herself down as Native American and was part of the school's federal affirmative action records until 2004.

But it was noted in 1989 she switched from identifying as white at the University of Pennsylvania, bringing her race in line with the AALS record, two years after starting her Penn role.

Warren told the Post the apology on Tuesday applied to her records for Harvard, Penn and AALS.

'She is sorry that she was not more mindful of this earlier in her career,' Warren campaign spokeswoman Kristen Orthman, said.

Sen. Elizabeth Warren apologized for a DNA testing stunt that established the slimmest of Native American blood in her heritage

On Monday it was revealed Warren apologized to the Cherokee Nation after releasing the results of a DNA test that showed a tiny sliver of her ancestry was Native American.

The liberal senator who is exploring a run for president 'reached out to us and has apologized to the tribe', said Cherokee Nation communications director Julie Hubbard.

'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,' said Hubbard. '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.'

In apologizing, Warren took a course of action that her advisers had counseled after her release of the DNA test results drew criticism and mockery from rivals rather than put to rest her long-running spat with President Trump over her claim of having some Native ancestry.

Warren's DNA analysis, conducted by a privately hired scientist who is an expert on Native American genomics, showed that her Native American ancestry was between 0.09 percent and 1.5 percent.

The low end of that scale equates to 1/1,024th.

In addition to failing to silence Trump, Warren's release of the information drew complaints from some native groups, who stress cultural heritage rather than scientific testing for tribal affiliation.

'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,' said Julie Hubbard, Communications Manager at Cherokee Nation

At the time of the release, Cherokee Nation Secretary of State Chuck Hoskin Jr. blasted the move.

'Using a DNA test to lay claim to any connection to the Cherokee Nation or any tribal nation, even vaguely, is inappropriate and wrong,' he said at the time, CNN reported. 'It makes a mockery out of DNA tests and its legitimate uses while also dishonoring legitimate tribal governments and their citizens, whose ancestors are well documented and whose heritage is proven.' A Warren spokesperson didn't comment.

In December, on the eve of her campaign launch, it was revealed that Warren's advisors were counseling her to issue a forceful apology for the bungled release.

Democrats blasted Warren in October for drawing attention to her biggest political liability in the middle of a tense midterm election cycle where Democrats were desperate to retake control of the House and break the GOP's lock on government control – an effort that succeded.

A landmark study published in 2014 in the American Journal of Human Genetics found that, on average, European-Americans have 0.18 per cent Native American blood – twice as much.

President Donald Trump has long mocked Warren's claim of being part Cherokee, calling her 'Pocahontas' and licking his chops at the prospect of facing her in 2020

Pocahontas was an Algonqiuan Indian princess who lived from 1595 to 1617

The New York Times reported last month that Warren worries she damaged her standing with Native American groups and other liberal blocs, especially minority constituencies, by willingly releasing her DNA results.

She now must compete among a slew of Democrats in an increasingly crowded presidential field. Just this week, Sen. Kamala Harris of California and Cory Booker of New Jersey launched presidential campaigns, and Vermont Sen. Bernie Sanders – who ran in 2016 when Warren took a pass – is considering a run.

'I put it out there. It's on the internet for anybody to see,' she told the Times of her DNA test findings. 'People can make of it what they will.'

Warren is a leader on the Democratic Party's left edge and a fierce critic of Wall Street whose potential as a Trump-spearing 2020 White House contender forced the long-simmering issue of her family history to the surface.

Warren (top right) is part of a growing field of potential Democratic presidential nominees that also includes (from top left) former Vice President Joe Biden, Senator Cory Booker, Senator Kamala Harris; and (from bottom left) Senator Sherrod Brown, Senator Bernie Sanders, Texas Congressman Beto O'Rourke and former New York City Mayor Michael Bloomberg

President Donald Trump began calling Warren 'Pocahontas' in the spring of 2016 in order to blunt her attacks as she threatened to become a key surrogate for his presidential rival Hillary Clinton.

'Goofy Elizabeth Warren, sometimes referred to as Pocahontas because she faked the fact she is native American, is a lowlife!' he tweeted as the Republican primary season wrapped up in May 2016.

Her ultimate response in October 2018 as Trump continued to taunt her during public rallies made some Democrats raise questions about her political agility.

Critics say Warren used a hint of Indian background to justify classifying herself as a minority during her law school teaching career, in order to gain preferential hiring treatment.

Law schools at Harvard and the University of Pennsylvania have disputed that claim.

Warren did, however, listed herself for 10 years as a minority in directories maintained by the Association of American Law Schools, beginning in 1986. That period of time stretched from the year before Penn hired her to the year after Harvard Law School lured her away.

Trump said of Warren (left) during an October campaign rally in Montana: 'She has so little Indian blood – well, she has none – that I cannot call her Pocahontas anymore'

She told reporters during her first Senate campaign in 2012 that, 'I listed myself in the directory in the hopes that might mean that I would be invited to a luncheon, a group, something that might happen with people who are like I am.'

Still, Trump has mocked her for years, calling her 'Pocahontas' and a 'fake Indian.'

The New England Historic Genealogical Society reported in 2012 that it had unearthed a 1894 document suggesting Warren's great-great-great-grandmother, O.C. Sarah Smith, was at least partially Native American.

It turned out the society had found a family newsletter that described a marriage license application, but hadn't seen that document.

The conservative Breitbart News website later tracked down the marriage license and certificate and found that a column on both marked 'race' was left blank.

Warren had de-emphasized her Cherokee lineage since winning her Senate seat in 2012, but the president's use of the issue to marginalize her was always expected to become a flashpoint if the two were to go head-to-head in 2020.

While Trump's rally crowds eagerly lap up his snarks about 'Pocahontas,' Trump has used the polarizing nickname in at least one cringe-worthy moment at the White House.

In November 2017 he slipped it into a speech during an event honoring Navajo 'Code Talkers' who had used their native language as an unbreakable secret code to protect American military communications during World War II.

Asked minutes later during a press briefing why the president would use 'offensive' language at such an event, White House Press Secretary Sarah Sanders shot back: 'I think what most people find offensive is Senator Warren lying about her heritage to advance her career.'