WASHINGTON (AP) — The Senate on Thursday defeated a GOP 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. "A lot of them don't have any consciences. They'll take it," Boxer said.

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.

Both parties were using the issue to rally their bases; Republicans sought to hold together conservatives and others in the midst of an unsettled battle for the presidential nomination. And for Obama, there is no constituency more crucial to his re-election chances than women.

In the end, the vote hung on a handful of centrists as Democrats chose a parliamentary maneuver that required only 50 votes to kill the amendment. Only three Democrats and one Republican defied their parties.

Voting with Republicans in favor of the amendment were Sens. Joe Manchin of West Virginia and Bob Casey of Pennsylvania, both up for re-election, and Ben Nelson of Nebraska, who is retiring. Sen. Olympia Snowe, R-Maine, who only this week abandoned her re-election bid out of frustration with the polarized Congress, was the lone Republican to vote to defeat the amendment.

Democrats pounced on the symbolism of Snowe's decisions. "If Republicans keep this up, they're going to drive away independent voters, women and men, just as they are driving moderates out of their caucuses," said Sen. Chuck Schumer, D-N.Y.

But Republicans kept one such senator in the fold on the vote, Snowe's fellow senator from Maine. Sen. Susan Collins kept all sides on edge until minutes before the vote, saying on the Senate floor that she was troubled that the administration could not assure her that faith-based self-insured organizations would be protected from the mandate to cover contraception.

"I feel that I have to vote for Sen. Blunt's amendment," she said.

Both sides protested strenuously that an issue affecting millions of Americans was being used for political gain, but the debate was steeped in election-year strategy. The presidency and the congressional majorities are at stake, and Obama's contraception coverage policy is one of several cultural issues that have become prominent in the nation's political discourse this year.

On the presidential campaign trail, GOP frontrunner Mitt Romney apparently stumbled over a question of whether he supports the amendment, in the end saying that he does support it, "of course." His main challenger, former Pennsylvania Sen. Rick Santorum, has said that contraception conflicts with this Roman Catholic beliefs.

Blunt, Romney's liaison to Congress, predicted the issue won't go away.

It "won't be over until the administration figures out how to accommodate people's religious views at it relates to these mandates," he told reporters after the vote. "This is a debate that might be settled at that building across the street," referring to ethe Supreme Court.

The Obama administration and congressional Democrats said Blunt's measure was so broad it could allow employers to opt out of virtually any kind of medical treatment.

"This proposal isn't limited to contraception nor is it limited to any preventive service. Any employer could restrict access to any service they say they object to," said Secretary of Health and Human Resources Kathleen Sebelius. "The Obama administration believes that decisions about medical care should be made by a woman and her doctor, not a woman and her boss."

A majority of Americans support the use of contraceptives. The public is generally in favor of requiring birth control coverage for employees of religiously affiliated employers, according to a CBS News/New York Times poll Feb. 8-13. The survey found that 61 percent favor the mandate, while 31 percent oppose it. Even Catholics, whose church strongly opposed the recent government mandate, support the requirement at about the same rate as all Americans.