By Dan Merica, CNN

In a sermon that likely broke the law, Indiana pastor Ron Johnson told his 400 congregants Sunday that for those who believe in the Bible, the decision to vote against President Barack Obama “is a no-brainer.”

“For Christian people who believe the Bible is the inspired world of God, it is not rocket science,” Johnson told CNN after his sermon.

Johnson’s anti-Obama sermonizing likely violated the so-called Johnson Amendment, an Internal Revenue Service rule that forbids churches that receive tax-exempt status from the federal government to intervene in “any political campaign on behalf of (or in opposition to) any candidate for elective public office.”

But Johnson appears comfortable with defying the IRS. His sermon was part of a national campaign by the Alliance Defending Freedom, a conservative Christian legal organization that has organized Pulpit Freedom Sunday since 2008, encouraging pastors to flout the Johnson Amendment with political endorsements from the pulpit.

Alliance Defending Freedom said that more 1,500 other pastors across the United States participated Sunday. The goal: to force the IRS to come down on these churches so the organization, whose network includes 2,200 attorneys, can test the Johnson Amendment’s constitutionality.

“The IRS has the ability and the authority to regulate their sermons. We are giving them the opportunity to do that, and if they challenge that, we will challenge that in court,” said Erik Stanley, Alliance Defending Freedom's senior legal counsel. “It is all about creating a test case to find the Johnson Amendment as unconstitutional.”

With less than a month until the presidential election, what was said at this year’s Pulpit Freedom Sunday could hold more sway than in previous years.

Critics say the movement is a Republican front dressed up as an exercise in religious freedom, an allegation the event organizer rejects.

“The ADF wants to elect the next president. They want to elect Mitt Romney,” said Barry Lynn, executive director of Americans United for Separation of Church and State. “This is not about some principle.”

Johnson denies that, noting on Sunday he did not endorse Romney, the Republican presidential nominee, but instead urged his congregation to vote against Obama, whose policies he called “un-American.”

He said the speech received a number of standing ovations.

When CNN asked to be put in touch with a church that plans to endorse the president, representatives from the organization said they don’t screen whom the churches plan to endorse.

The Alliance Defending Freedom has ties to other conservative Christian groups such as the American Family Association and Focus on the Family.

“I think there is a possibility that in some of these mega-churches, a pastor's saying it is OK to vote for Mitt Romney … could increase voter turnout,” Lynn said.

So far, the effort has received little to no response from the IRS.

The IRS did not respond to CNN’s request for comment.

Many of the sermons from Sunday will be sent to the nation’s tax collection agency, a move that organizers hope will make it easy for the IRS to come down on the churches. According to Stanley, the majority of the messages in past years have gone unnoticed, and only a handful of pastors receive letters, some of which threaten to revoke the churches' tax-exempt status.

This nonenforcement by the IRS has emboldened some pastors and the Alliance Defending Freedom, said Lynn of Americans United. According to pastors who have participated in the past, the fact the IRS rarely if ever comes down on these churches encourages them to keep endorsing.

Stanley and the Alliance Defending Freedom theorize that the IRS doesn’t want to be challenged in court and that the agency may be disorganized.

But the lack of enforcement stems from bureaucratic uncertainty about what rank an IRS official must be to initiate an investigation, Lynn said.

In the past, the IRS has investigated churches that it suspected of violating the Johnson Amendment.

Four days before the 1992 presidential election, the Landmark Church in Binghamton, New York, ran a full-page ad in USA Today that said, "Christians Beware," followed by a list of Bill Clinton's positions on homosexuality, abortion and the distribution of condoms. At the bottom, the church asked for donations to help pay for the ad.

According to Lynn, Americans United filed a complaint, and the church lost its tax-exempt status in 1995.

Landmark Church pastor Dan Little took the IRS to court, arguing the agency was violating the church's First Amendment rights and the agency was only able to revoke the tax-exempt status of a "religious organization," not an actual church.

Both a U.S. District Court judge and a federal appeals court rejected those arguments.

Johnson, the Indiana pastor, laughs when asked about those who question whether a pastor should be allowed to endorse from the pulpit.

“Pastors understand the so-called separation of church and state, as it is currently understood. We understand how marginalized we are becoming,” Johnson said. “We are supposed to be part of the community discussion about issues that matter.”