US Catholics have strongly criticised a White House rule that would make Church-linked schools and hospitals give employees access to birth control.
Catholic League head Bill Donohue said it would be fought with lawsuits and "maybe even in the streets".
Under President Barack Obama's healthcare reform, employers must offer insurance that includes contraceptives.
A senior White House adviser said the president wanted to find a solution and was not targeting any religion.
"I'm less concerned about the messaging of this than finding a resolution that makes sense," White House adviser David Axelrod said.
"I think we need to lower our voices and get together."
The regulation - issued by Health Secretary Kathleen Sebelius as part of the new health care law - goes into effect more than a year from now.
Churches were given a waiver under the new law, but institutions like Catholic universities and hospitals are not exempt.
Key Catholic bishops have added their voices to the chorus of disapproval.
"Coercing religious ministries and citizens to pay directly for actions that violate their teaching is an unprecedented incursion into freedom of conscience," Archbishop Timothy Dolan of New York wrote in an editorial for the Wall Street Journal.
Dr Dolan was joined by bishops in Connecticut and Massachusetts in arguing that the rule infringed on Catholic religious belief by forcing institutions to pay for contraceptive drugs.
The US Conference of Catholic Bishops has asked Catholics to protest against the rule by writing to their members of Congress.
Letters from the leadership condemning the policy were reportedly read in churches across the US during services on 29 January.
Former Massachusetts Governor Mitt Romney, who is running for the Republican nomination for president, has also come out publicly against the policy.
On the stump in Colorado, ahead of caucuses there on Tuesday, he described the policy as a "violation of conscience". Earlier, he urged his followers on Twitter to add their names to a petition against the rule.
"The Obama administration is at it again," Mr Romney wrote in the introduction to the petition. "They are now using Obamacare to impose a secular vision on Americans who believe that they should not have their religious freedom taken away."
The rule does not affect existing conscience clauses for doctors.
The US health secretary, who announced the decision last month, defended it in an editorial in USA Today on Sunday.
"This is not an easy issue," Ms Sebelius wrote, adding: "We are working to strike the right balance between respecting religious beliefs and increasing women's access to critical preventive health services."
When asked about the new rule, White House Press Secretary Jay Carney told reporters on Monday that "those institutions where women of all faiths, many faiths work need to have the same kind of coverage that all other American women have".
A recent poll suggested that 52% of American Catholics believe religiously affiliated colleges and hospitals should have to provide coverage that includes contraception.