Mr. Trump has slurred Ms. Warren for years over her ancestry claims, and the personal attacks on her come out of the Trump playbook from 2016 that targeted both Republicans and Mrs. Clinton. Michael Steel, a Republican strategist who was an adviser for former Gov. Jeb Bush’s 2016 presidential campaign, said Mr. Trump would need to “tear down” whoever is the eventual Democratic nominee in order to win re-election, given his low approval ratings.

“One of Senator Warren’s great strengths is her ability to connect her biography to her record to her plans for the future,” he said. “That gets undermined if she can’t rely on her biography as the foundation of that progression.”

All year, Ms. Warren has framed her personal story to voters as one of perseverance, focusing on her upbringing in Oklahoma, her family’s financial struggles and her winding path to an eventual career as a law professor. Voters often praise her biographical story as a powerful part of her pitch, which centers on bringing about big change to an economic and political system that she describes as rigged against ordinary people.

As part of that story, Ms. Warren tells crowds that at the end of her first year as a public-school teacher, she was “visibly pregnant” and lost her job. A retired teacher from Ms. Warren’s school in New Jersey told CBS News, “The rule was at five months you had to leave when you were pregnant.”

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Teachers in the United States were commonly pushed out of their jobs during pregnancy at the time, either through termination or the requirement of an unpaid leave of absence. Congress passed the Pregnancy Discrimination Act in 1978, which barred employers from treating pregnant women differently from other people “similar in their ability or inability to work,” but claims of discrimination persist to this day.

“These attacks on Senator Warren are absurd,” said Senator Kirsten Gillibrand of New York, who dropped out of the presidential race in August. “Of course pregnancy discrimination was a serious problem in 1971, and it continues to be a widespread issue today despite a law banning it. The stories that women are sharing are real and powerful.”

Regardless, the Republican National Committee dismissed Ms. Warren’s description of losing her job, citing a 2007 interview in which she discussed her public-school teaching career but did not mention being forced out, as well as records showing that the school board had approved a contract for Ms. Warren for the next school year.