U.S. lawmakers have narrowly rejected a bid to reverse a part of President Barack Obama's health care reform law that focused on birth control.

The Democratic-controlled Senate defeated a Republican measure by a vote of 51-48. It sought to allow employers and insurers to opt out of parts of the law they found morally objectionable.

The focus was a provision of the health care reform law that requires almost all employers who provide health insurance to cover the cost of women's contraception.

Religious groups, including Roman Catholic bishops in the United States, have strongly objected to the law, saying it would force them to violate their beliefs and provide birth control for employees at religiously affiliated institutions, like hospitals and universities.

Supporters of the amendment said no religious organization should be forced to violate the tenets of their faith. Opponents said the amendment would have allowed employers and insurers to deny coverage for any medical treatment for any reason.

Last month, President Obama sought to ease controversy over the birth control requirement by announcing religiously-affiliated institutions would not be forced to pay for contraception. Instead, he said health insurers will be required to offer women free birth control directly when their employers have religious objections.