A debate is raging in Congress over Planned Parenthood after controversial videos released this summer showed several of the organization's employees discussing the practice and logistics of fetal tissue donation.

The footage, edited by anti-abortion activists, spurred calls among conservatives to halt the flow of taxpayer money to Planned Parenthood, which annually receives about $500 million in public dollars.

Three weeks before lawmakers must pass new legislation to fund the government, at least 28 Republicans, all men, have vowed to vote against any bill that contains support for Planned Parenthood, causing concern about the possibility of another shutdown.

Defunding Planned Parenthood, however, would have broad ramifications, especially for low-income women who rely on subsidized services for birth control.

The Guttmacher Institute, a nonprofit reproductive health organization, released a report this week breaking down how often these women turn to the organization for reliable contraceptives. Many would have to leave town to find birth control if the resource were to evaporate, the data implies.

Money from the government is Planned Parenthood's largest source of funding. Eighty percent of clients receive “services to prevent unintended pregnancy,” according to the organization's data, and 3 percent of the centers’ annual services are abortions:

In response to a request from the Congressional Budget Office, the Guttmacher staff analyzed data from its 2010 Contraceptive Needs and Services 2010, the latest available information. Some highlights:

1. In two-thirds of the 491 counties with a location, Planned Parenthood serves at least half of all American women who receive birth control from safety-net clinics.

In one-fifth of these counties, a Planned Parenthood site is the only family planning center for clients who rely on subsidized help, the report found.

The centers serve more than 40 percent of women who receive birth control from safety-net providers in 18 states and more than half of such women in 11 states.

2. Most women in the United States who say they need publicly supported contraceptives live in counties with a Planned Parenthood center.

Nearly two-thirds of these 19 million women, who are sexually active and have a family income below 250 percent of the federal poverty level, are a quick trip away from a Planned Parenthood center. Thirty percent live in counties where Planned Parenthood serves most women who fit this description.

3. Planned Parenthood serves a bigger share of safety-net birth control clients than other types of providers that offer care to low-income women.

Planned Parenthood makes up 10 percent of publicly supported family planning centers, according to Guttmacher, but they serve 36 percent of clients who seek out the service. Centers run by health departments, for comparison, offer care to 27 percent of these clients. Hospital sites serve 8 percent.

Planned Parenthood also covers more clients per year than other safety-net providers. The organization annually serves an average of 2,950 women who request birth control. Hospitals, by contrast, provide the services to an average of 770 contraceptive clients, Guttmacher reports. Health departments see an average of 750.