On Oct. 2, hundreds of Christian pastors are expected to engage in a mass act of civil disobedience across the United States. From their Sunday pulpits, they plan to purposely violate a federal law by openly opposing or endorsing political candidates.

Why break the law on purpose?

These pastors, most of them evangelical, are part of an effort to bait the Internal Revenue Service into fining one of the churches or taking away its tax-exempt status. If the IRS takes the bait, then that case will be challenged all the way up to the Supreme Court in hopes of overturning the law.

The law bars houses of worship and other types of nonprofits from participating in political campaigns on behalf of – or in opposition to – any candidate for public office. It was passed in 1954 with help from then-Sen. Lyndon Johnson after nonprofit groups nearly defeated him in an election. Up until then, American churches were free to align with politicians – or rail against them.

The group pushing this legal test, the Alliance Defense Fund, has been enlisting more and more pastors since 2008. The yearly autumn protest, called Pulpit Freedom Sunday, is timed just before elections. Many participating pastors are eager to turn the US into “a Christian nation” by playing a direct role in politics.

With the 2012 presidential campaign heating up, and many Evangelicals gearing up to defeat President Obama and his allies, the event is expanding fast. The next one, in 2012, will be just weeks before that crucial vote. It could make a big difference in the ballot counts of key electoral states such as Florida or Virginia.

Most religious groups (such as the one that publishes The Christian Science Monitor) do not endorse candidates – many of their followers would heartily protest. And they avoid direct partisan politics – other than taking stands on issues, which is legal under the law – in order to not become entangled with political parties and the mudslinging that comes with them.

Drawing this fine line between church and state, or between spiritual matters and electioneering, is a protection from government intrusion on organized religion. And it helps political parties from being controlled by a particular religion – something the Muslim world is actively debating, even killing over.

Still, many religious leaders, not just Evangelicals, do often cross this fine line.

A group of New York City rabbis, for example, advised Jews not to vote for the Democratic candidate in a Sept. 13 special election to fill an empty seat for the US House. And leaders of many black churches often invite candidates to speak to their congregations, grilling them on their views.

The wisdom in churches not endorsing candidates, however, can be a separate matter from difficult constitutional questions. If the IRS acts after this Sunday, it is uncertain how the high court might rule.

The IRS has acted against churches only a few times under the law. And in only one case (involving a church that had run an ad against Bill Clinton) was there a ruling by a federal appeals court – upholding the law.

As background, the Alliance Defense Fund argues that no house of worship should ever be taxed by government. And people who donate money to a church should be able to deduct such gifts on their income taxes.

It is those donations that are at the heart of the constitutional battle.

How can government not tax such gifts when they are used by churches for partisan political activities? After all, donations given to nonreligious groups also engaged in politics are taxed. Are politicized churches more deserving of a tax break?

At the same time, do Americans want the IRS checking the sermons of church leaders suspected of endorsing candidates? Might that be a violation of both free speech and the constitutional provision against government meddling in religion?

The IRS says it looks for only political speech, not religious creed. And churches are free to set up a separate organization – whose donations are taxable – that can be a partisan group. Pastors, too, can endorse candidates outside a church setting.

The 1954 law also applies equally to many types of nonprofits and does not place an unfair or substantial burden on only religious groups – a key constitutional point.

Sorting out the competing issues, however, won’t be easy if the issue gets to the high court. Even harder is the need to persuade all religious groups to stay clear of directly intervening in elections – so that such a law wouldn’t even need to exist.

People of faith do need to be involved in public affairs – to vote, to pray, and even to individually endorse candidates. But if an organized religion wants to be held separate enough from government and not be taxed, it ought not morph into something else, like a political machine, come every election.