WASHINGTON — Supporters of a woman's right to abortion pushed back hard Thursday after a major breast-cancer charity halted funding to Planned Parenthood, the biggest single abortion provider in the United States.

The very public split between longtime partners Planned Parenthood and the Susan G. Komen for the Cure foundation foretold how abortion could be a pivotal issue in November's US presidential and congressional elections.

More than $400,000 has so far been pledged online by 6,000 donors to Planned Parenthood, on top of $250,000 from Texas oil baron Lee Fikes and his wife Amy to set up a Planned Parenthood Breast Health Fund.

"People respond powerfully when they see politics interfering with women's health," a spokesman for Planned Parenthood, which offers both abortions and breast screenings for less well-off American women, told AFP.

"That's why we've seen a tremendous outpouring of support."

New York's billionaire mayor Michael Bloomberg offered to match further donations to Planned Parenthood up to a maximum $250,000. "Politics have no place in health care," he said in a statement.

Online, political action group MoveOn.org gathered 280,000 names for a protest petition.

"It's incredibly disappointing for an organization founded on protecting women's health to play politics with real women's lives," it said.

In Washington, 26 Democratic and independent senators fired off a letter to Komen, urging it to reconsider its move.

"It would be tragic if any woman -- let alone thousands of women -- lost access to these potentially life-saving screenings because of a politically motivated attack," they said.

Abortion has been legal across the United States since the Supreme Court's landmark Roe versus Wade ruling 39 years ago, which declared it was a private matter between a woman and her doctor.z

But the pro-life camp remains a powerful force, with 45 percent of respondents in an ABC News/Washington Post poll last year saying abortion should be illegal in most or all cases.

"Not surprisingly, Planned Parenthood and its allies are on the warpath," said the conservative Family Research Council. "Pro-lifers, meanwhile, are tickled 'pink' that Susan G. Komen has changed to an abortion-neutral stance."

Komen was founded in 1982 by Nancy Brinker, a prominent Republican and US ambassador to Hungary under former president George W. Bush, in memory of her sister, who died of breast cancer at age 36.

It famously came up with the pink-ribbon logo of the worldwide quest for a breast-cancer cure, and for years it helped underwrite breast screening at Planned Parenthood clinics, including $680,000 last year.

In a statement Wednesday, Planned Parenthood alleged that "anti-choice groups" had been leaning hard on Komen to cease funding.

"We are alarmed and saddened that the Susan G. Komen for the Cure Foundation appears to have succumbed to political pressure," Planned Parenthood president Cecile Richards said.

"Our greatest desire is for Komen to reconsider this policy and recommit to the partnership on which so many women count."

In a video posted on YouTube, Brinker denounced "scurrilous rumors" and "mischaracterizations" of what she called Komen's decision to tighten up its grant-giving process and avoid "duplicative" contributions.

"We will never bow to political pressure," added Brinker, now a goodwill ambassador for the World Health Organization. "We will always stand firm in our goal to end breast cancer."

Planned Parenthood, which offers abortions for as little as $300, said it carried out nearly 330,000 abortions in 2009, representing about three percent of all its services.

Most of its nearly three million patients live below the poverty line, it said, adding that it also performed 748,000 breast exams.

Planned Parenthood is the top US abortion provider, but most of the nation's more than one million abortions every year take place in clinics and private doctor's offices, the National Abortion Federation says.

In November, the pro-life camp suffered a setback when a proposal to grant "personhood" rights to unborn babies in Mississippi -- effectively making abortion illegal in the Bible Belt state -- was soundly defeated by voters.