NEW YORK -- Conservative legal advocates are recruiting pastors nationwide to defy an IRS ban on preaching about politicians, in a challenge they hope will abolish the restriction.

The Alliance Defense Fund, based in Scottsdale, Ariz., will ask the clergy to deliver a sermon about specific candidates Sept. 28. If the action triggers an IRS investigation, the legal group will sue to overturn the federal rules, which were enacted in 1954.

Under the IRS code, churches can distribute voter guides, run voter registration drives, hold forums on public policy and invite politicians to speak at their congregations.

However, they cannot endorse a candidate, and their political activity cannot be biased for or against a candidate, directly or indirectly.

The Alliance Defense Fund said Friday that the regulations amount to an unconstitutional limit on free speech and government intrusion into religion.

"It certainly does have a chilling effect," said Mike Johnson, senior counsel for the fund. "I think that there is a lot of fear and intimidation and disinformation about the parameters that do exist."

Johnson said about 100 pastors have expressed interest in participating so far.

The IRS has stepped up monitoring of nonprofit political activity during the 2008 election. Punishments can range from a financial penalty to loss of tax-exempt status.

IRS investigations are confidential and the agency does not discuss the cases.

However, the United Church of Christ, which counts Sen. Barack Obama as a member, has said that it is under IRS review because of a speech given by the Democratic presidential candidate at the denomination's national meeting last year.

Americans United for Separation of Church and State, an advocacy group in Washington, monitors church political activity and consistently files complaints with the IRS. They said Friday that they will notify the agency of any pastor who participates in the ADF campaign.

Some religious groups support keeping politics out of the pulpit.

J. Brent Walker, executive director of the Baptist Joint Committee for Religious Liberty in Washington, which advocates for religious freedom, said churches should be involved in public issues, but partisan activity can "compromise the essential calling to spread the Gospel."

"The church can't raise prophetic fist at a candidate or at a party," Walker said, "when it's locked up in a tight bear hug with that candidate or party."