* Senate votes 51-48 against Blunt's contraceptives measure
* Amendment aimed to reverse Obama contraceptives rule
* Republicans oppose policy on religious, moral grounds (Adds details)
By David Morgan and Thomas Ferraro
WASHINGTON, March 1 (Reuters) - The U.S. Senate narrowly backed a key plank of President Barack Obama's healthcare law on Thursday by rejecting a sweeping Republican measure that would have allowed employers to opt out of birth control coverage and other services on moral grounds.
Senators voted 51-48 to set aside a measure proposed by Republican Roy Blunt that would have exempted employers like Catholic hospitals, universities and charities from an Obama healthcare provision that requires most employers to offer free insurance coverage for women's contraceptives.
Democrats and some Republicans had warned that Blunt's "Respect for Rights of Conscience Act," introduced as an amendment to an unrelated highway bill, contained language broad enough to deny any number of benefits from prenatal care and childhood vaccinations to cancer screenings including mammograms on the basis of a conscientious objection.
"It would allow any employer or insurer to deny coverage for virtually any treatment for virtually any reason," said Senate Majority Leader Harry Reid, who denounced the measure as "an extreme, ideological amendment."
The Senate debate provided a national stage for a growing American culture war between women's reproductive rights and conservative social values, incendiary issues that both Democrats and Republicans hope to exploit for votes in the Nov. 6 election.
Maine Senator Olympia Snowe, the only Republican to cross party lines and vote against the Blunt amendment, made it clear before the vote that she favored a "conscience clause" for those opposed to contraceptives but had misgivings about the bill's wording, which did not mention birth control. Snowe announced earlier this week that she would not seek a fourth term, saying she had grown tried of destructive partisan battles.
The Blunt measure would have amended a section of the 2010 Patient Protection and Affordable Care Act that is designed to set national standards for essential healthcare benefits for the first time, including a host of preventive services.
Three Democrats with conservative records on social issues - Ben Nelson, Joe Manchin and Robert Casey - joined Republicans in voting for the amendment.
In an attempt to quell an uproar over contraceptives, Obama announced this year that religiously affiliated employers would not be required to offer free birth control to workers, saying the onus would fall instead on insurers. The president's "accommodation" is expected to be formulated into legal language and published as a proposed rule soon.
Neither Obama's gesture, nor Thursday's vote, will likely resolve the controversy. Catholic authorities who view artificial conception as a sin and want the policy rescinded.
"This issue will not go away unless the administration decides to take it away," Blunt said.
A Baptist who entered the Senate in early 2011, Blunt built a staunch conservative record on social issues including partial-birth abortion, same-sex marriage and gay adoption during a 14-year career in the House of Representatives. He has earned high ratings from Catholic organizations.
Catholic bishops who are at the forefront of the opposition also described Thursday's outcome as only a temporary setback.
"We will not rest until the protection of conscience rights is restored and the First Amendment is returned to its place of respect in the Bill of Rights," said Bishop William Lori of Bridgeport, Connecticut, who chairs a committee on religious liberty for the U.S. Conference of Catholic Bishops.
A Kaiser Family Foundation survey of 1,500 adults released on Thursday showed that nearly two-thirds of Americans favored Obama's policy, including clear majorities of Catholics and evangelicals.
Blunt's amendment was never expected to pass the Democratic-controlled chamber. But it gave Democrats and Republicans an opportunity for election-year posturing on an issue that has spawned opposition from Catholics and Protestant evangelicals including a number of opposition lawsuits.
In a federal lawsuit in Nebraska, seven U.S. states have joined with Catholic groups and individuals to oppose the rule.
A different battle in the same war erupted in Congress where Democratic lawmakers called on the House Republican leadership to repudiate conservative talk radio host Rush Limbaugh. Limbaugh on Wednesday described a female student from a Catholic university of being "a slut" and "a prostitute" after she spoke openly about the need for birth control coverage.
Women's issues have been front-and-center in recent months.
Just over a month ago, breast cancer charity Susan G. Komen for the Cure became engulfed in controversy for cutting funds to Planned Parenthood, an abortion provider, and then rescinding the decision.
And Health and Human Services Secretary Kathleen Sebelius faced accusations of bowing to political pressure from social conservatives for deciding late last year to make the morning-after pill called Plan B One available without a prescription only to women older than 17.
Democrats sought to frame the Blunt amendment as a Republican attack on women's access to healthcare, an argument that lawmakers hoped would appeal to independent women.
"Vote down this dangerous measure," California Democrat Barbara Boxer urged her colleagues. "Vote it down. Stand for the women in the families of this nation."
Republicans presented a religious liberty argument that could resonate with Catholics and other social conservatives.
"If there is one good thing about this debate, it has given us all an opportunity to reaffirm what we believe as Americans," Senate Minority Leader Mitch McConnell said.
Even before the vote got under way, the Obama re-election campaign weighed in with the charge that nearly 80 million women who receive coverage through their employers could lose access to preventive services under the Republican amendment.
A statement from Obama deputy campaign manager Stephanie Cutter said the public could thank Republican presidential hopefuls Mitt Romney and Rick Santorum for "helping to pave the way for this anti-contraception agenda."
The statement also referred to Romney's muddled position on the Blunt amendment. Romney told an interviewer on Wednesday that "I'm not for the bill." A spokeswoman said later he was confused by the way the question was posed and that Romney supported the amendment. (Additional reporting by Anna Yukhananov in Washington, and Stephanie Simon in Denver; Editing by Mary Milliken, Peter Cooney and Sandra Maler)