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Planned Parenthood, which has become an ideological minefield in the 2016 presidential election, said Thursday that it would endorse Hillary Clinton — its first endorsement in a presidential primary in the nonprofit’s 100-year existence.

Mrs. Clinton will officially accept the group’s support Sunday at a campaign rally in Manchester, N.H. The decision to break with tradition and endorse Mrs. Clinton comes as the House has approved a measure, endorsed by the leading Republican presidential candidates, that would repeal parts of the Affordable Care Act and strip away federal financing for Planned Parenthood, which provides reproductive and health care services.

“Everything Planned Parenthood has believed in and fought for over the past 100 years is on the ballot,” said Cecile Richards, president of Planned Parenthood.

In a statement, Mrs. Clinton said she was “honored” by the endorsement and called the bill to defund the group “a jarring reminder of what’s at stake in 2016,” adding that as president she would “defend against attacks on reproductive health care, and protect access to affordable contraception and safe and legal abortion across the country.”

The endorsement, technically made through the nonprofit’s advocacy arm, the Planned Parenthood Action Fund, brings with it at least $20 million to spend in this election cycle on presidential and Senate races in crucial battleground states, including New Hampshire, Ohio, Pennsylvania and Wisconsin

Other groups that support abortion rights, including NARAL Pro-Choice America PAC, have already endorsed Mrs. Clinton. But in Planned Parenthood’s case, Mrs. Clinton’s future and the group’s are intrinsically linked: Planned Parenthood needs to have a Democrat elected president to protect its funding, and Mrs. Clinton is hoping abortion rights and the Republican candidates’ positions will motivate female voters to support her.

In almost every speech, Mrs. Clinton receives some of her most rousing applause with the line: “I am for a woman’s right to choose, and I will fight against efforts to defund Planned Parenthood.”

The Clinton campaign has functioned almost as a marketing arm for Planned Parenthood, featuring a section on its official website titled “17 times Hillary Clinton stood with Planned Parenthood,” Facebook messages and Instagram posts with the hashtag #StandwithPP. (Ms. Richards’ daughter works on the campaign’s staff in Iowa.)

The strategy has merits. While Republican promises to restrict abortion rights resonate with conservative voters, polls show that access to legal abortion is one of the most motivating issues for female voters, particularly in critical swing states.

Half of male voters and 60 percent of female voters said Planned Parenthood should receive money from the federal government, according to a New York Times-CBS News poll conducted in September. The poll found that 72 percent of voters, regardless of gender, said they believed abortion should be generally available or available with limits.

The endorsement does not come without risks for Mrs. Clinton. Planned Parenthood is a polarizing topic and the group suffered damaging public relations setbacks this summer when anti-abortion rights activists released video of an official from the group discussing the price of providing fetal parts. Mrs. Clinton promptly called the video part of a “concerted attack” against “a woman’s right to choose.”

Conservatives accused the group of profiting from the sale of fetal tissue (a charge Ms. Richards called “categorically untrue”) and threatened to shut down the government rather than continue federal funding for the group. Republican presidential candidates, most notably the lone woman in the Republican field, Carly Fiorina, seized on the controversy to denounce the Planned Parenthood’s videos in visceral terms.

Both Mrs. Clinton and Planned Parenthood have tried to remind voters that the group’s clinics around the country provide more than reproductive services to nearly three million mostly low-income and minority patients. “For many women, we are their only doctor,” Ms. Richards said.

At the rally on Sunday in Manchester, Natarsha McQueen, a 40-year-old Planned Parenthood volunteer from Brooklyn, N.Y., is set to tell her story of detecting breast cancer at a local Planned Parenthood clinic.

“If you aren’t in the same economic state or haven’t been to our communities, you can’t relate to how important this is to women of color,” Ms. McQueen said.

Mrs. Richards said the Planned Parenthood Action Fund sent endorsement surveys out to all the presidential candidates on both sides, but only heard back from Democrats. She said Mrs. Clinton’s record on women’s issues and health care gave her an edge against her Democratic rivals, including Senator Bernie Sanders of Vermont.