Barbara Bush, whose father and grandfather were the last two Republican presidents before Donald J. Trump, said she was backing his opponent, Vice President Kamala Harris.

Ms. Bush, one of former President George W. Bush’s twin daughters, knocked on doors for Ms. Harris on Sunday in Bucks County, Pa., a politically purple area north of Philadelphia, according to the Harris campaign.

In a statement, Ms. Bush said she was “hopeful after speaking to so many voters about the important issues at stake in this election, including protecting women’s rights.”

The Harris campaign distributed an image of Ms. Bush smiling outside a brick home and wearing a white and blue hat emblazoned with the word “Kamala.”

People magazine previously reported on Ms. Bush’s endorsement of Ms. Harris.

Mr. Bush, the 43rd president, does not plan to endorse a presidential candidate, according to his spokesman, Freddy Ford. Mr. Ford said Mr. Bush had no comment on his daughter’s support for Ms. Harris.

Mr. Bush’s vice president, Dick Cheney, has also endorsed Ms. Harris, and Mr. Cheney’s daughter Liz, a former Republican representative from Wyoming, has served as one of the Harris campaign’s chief ambassadors in its efforts to reach suburban Republicans.

Mr. Trump, who beat and sometimes insulted Mr. Bush’s brother Jeb, the former governor of Florida, in the bruising 2016 Republican presidential primary, has been criticized in the past by members of the Bush family.

Jeb Bush once called Mr. Trump a “jerk.” Ms. Bush’s grandfather President George H.W. Bush, who died in 2018, was said to have once called him a “blowhard.”

And Ms. Bush’s grandmother, the first lady Barbara Bush, told CBS News in 2016 that she did not know “how women can vote for someone” like Mr. Trump, citing negative comments he had made about women.