Ms. Warren, Democrat of Massachusetts and a former professor at Harvard Law School, listed herself as a member of a minority group in a law school directory but has not claimed to be a Native American since being elected to the Senate in 2012.

Beyond Mr. Trump’s ridicule, some American Indian activists have pressured Ms. Warren to be more straightforward about her heritage and to also be a more aggressive advocate for the tribes, some of which account for the most impoverished communities in the country.

Such demands, combined with the attention that Mr. Trump commands with his mockery, prompted her to try to engage in what one supporter of hers allowed was something of a deck-clearing going into her expected re-election in November and before a Democratic primary race that will effectively start at the end of the year.

To that end, Ms. Warren, an Oklahoma native, used a recounting of her roots to not just tell her family’s ethnic story but to present a tale of Dust Bowl…

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