Defying a federal tax law they consider unjust, 33 ministers across the country will take to their pulpits this Sunday and publicly endorse a candidate for president.

They plan to then send copies of their sermons to the Internal Revenue Service, hoping to provoke a challenge to a law that bars religious organizations and other nonprofits that accept tax-deductible contributions from involvement in partisan political campaigns.

The protest, called Pulpit Freedom Sunday, was organized by the Alliance Defense Fund, a consortium of Christian lawyers that fights for conservative religious and social causes. When the fund first announced the protest this year, it said it planned to have 50 ministers taking part. As of Thursday it said it had hundreds of volunteers, but had selected only 33 who were fully aware of the risks and benefits.

The fund did not make the list of participants public, saying that it had received phone calls threatening to disrupt the sermons. One participant reached by telephone said he could not talk about it.