WASHINGTON — The Senate today defeated a Republican effort to roll back President Barack Obama's policy on contraception insurance coverage in the first vote on an issue that raised questions of religious and women's rights and riled Americans in this volatile election year.

The 51-48 vote killed an amendment that would have allowed employers and insurers to opt out of portions of the president's health care law they found morally objectionable. That would have included the law's requirement that insurers cover the costs of birth control.

Democrats said the measure would have allowed employers and insurers to opt out of virtually any medical treatment with the mere mention of a moral or religious objection.

“We have never had a conscience clause for insurance companies,” said Sen. Barbara Boxer, D-Calif.

The measure would have given insurers more opportunities to deny coverage for certain treatments, she added.

Republicans argued that the law needs to be reversed because it violates the First Amendment's guarantee of religious freedom by forcing insurers and employers to pay for contraception even if their faith forbids its use. Democrats said the amendment, sponsored by Sen. Roy Blunt, R-Mo, was an assault on women's rights and could be used to cancel virtually any part of the law.