Under constitutional protections of free speech and separation of church and state, churches are free to speak on any issue. But they risk losing tax breaks worth $145 billion in the past decade if they violate Internal Revenue Service rules by promoting or opposing any particular candidate. Other non-profits also have special tax status.

Citizens for Responsibility and Ethics in Washington, a political watchdog group, in its complaint to the U.S. Internal Revenue Service, cited reports of individual bishops "abusing their positions to advocate against the election of President Barack Obama."

The group's executive director, Melanie Sloan, said some bishops went too far by saying a vote for Democrats would mean going to hell. "I don't think the Catholic bishops should be intimidating parishioners to advocate for any particular candidate," said Sloan.

The Wisconsin-based Freedom from Religion Foundation complained to the IRS about possible illegal political campaign intervention by Wisconsin Catholic bishops and the Charlotte, North Carolina-based Billy Graham Evangelistic Association.

IRS spokesman Dean Patterson declined to comment on the complaints or on whether there was any investigation. "Federal law prohibits the IRS from discussing specific taxpayers or situations," Patterson said.

The U.S. Conference of Catholic Bishops, through its spokeswoman Sister Mary Ann Walsh, said it would not comment on what a bishop says in his diocese.

The Billy Graham group said that its newspaper ads in publications like the Wall Street Journal and USA Today advocated votes for candidates who support "biblical values" but mentioned no candidate or party.

The ads, signed by Graham, asked voters to back candidates who support "the biblical definition of marriage between a man and woman" and who protect "the sanctity of life," an apparent reference to the group's opposition to abortion.

The conference of bishops waged a campaign this year against the Obama administration's health care requirement birth control be covered by health insurance.

Church doctrine is opposed to contraception as a means of birth control. Church leaders also spoke out against same-sex marriage but were on the losing side in four states where the issue was on the ballot.