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President Donald Trump signed an executive order Thursday aimed at weakening the enforcement of a law that bars churches and tax-exempt groups from endorsing political candidates. A look at the law in question, known as the Johnson Amendment:

WHAT IT DOES

The law prohibits tax-exempt charitable organizations such as churches from participating directly or indirectly in any political campaign to support or oppose a candidate. That means no donations to candidates' campaigns and no public statements explicitly on behalf of or against a candidate.

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WHAT IT DOESN'T DO

It doesn't stop religious groups from weighing in on public policy or organizing in ways that may benefit one side in a campaign. Plenty of religiously grounded organizations or movements — Roman Catholic bishops, the Christian Coalition, you name it — have delved fiercely into political causes, and preachers of the left and right are not shy about exhorting their followers to political action. The law requires them to stop short of endorsing candidates, but their leanings are often not a mystery.

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WHY IT MATTERS

To supporters, the law is central to the constitutional separation of church and state. To opponents, it's a gag on the constitutional guarantee of freedom of expression.

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WHAT TRUMP DID

Trump said his action is meant to ensure that people are not penalized for their "protected religious beliefs" and that religious institutions are not "unfairly" targeted for political speech. In effect, the order discourages the IRS from going after churches aggressively for their political expression. But the law stays on the books; an executive order cannot change it.

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GENESIS: IN THE BEGINNING ...

A Republican Congress and Republican president, Dwight Eisenhower, brought the law into effect in 1954, but it was the handiwork of Lyndon Johnson, then a Democratic senator from Texas and later president.

LBJ was no altar boy for the Constitution's edicts on church and state. He was livid that a few nonprofit groups attacked him as a communist in a Senate campaign. His amendment was meant to slam such organizations by revoking their tax-exempt status if they went over the line in the partisan fray. His amendment was not focused on religious groups, but covers them.

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FITFULLY ENFORCED

The IRS has not been particularly aggressive in enforcing the law. Revoking a religious organization's tax exemption risks accusations that the government is crushing religious freedom. But the fact it possesses that power has kept interference in partisan politics in check, as the law's supporters see it, and represents government overreach in the mind of critics.

In September, Gov. Matt Bevin, R-Ky., told preachers the law was a "paper tiger" and they should embrace political speech more boldly. "There is no reason to fear it," he said. "There is no reason to be silent."

Lisa Runquist, a California lawyer who specializes in nonprofit and religious tax law, said it is a law with a lot of gray area, but she counsels clients to stay on the safe side of it. "Do you want to be the test case that goes forward?" she said she asks them.

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WHAT A VIOLATION LOOKS LIKE

Runquist cites a case in the 1990s when a New York church bought full-page newspaper advertisements that called on Christians not to vote for Bill Clinton. The organization Americans United for the Separation of Church and State lodged a complaint with the IRS. The IRS investigated, denied the church's exempt status and a federal court upheld that decision.

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Associated Press writers Dylan Lovan and Claire Galofaro in Louisville, Kentucky, contributed to this report.