Donald Trump, right, and Jerry Falwell Jr. in Sioux City, Iowa, on Jan. 31. (Dave Kaup/Reuters)

IN A rare moment of specificity during his speech to the Republican National Convention, Donald Trump called for repeal of the Johnson Amendment, a 62-year-old federal law that prohibits tax-exempt religious, scientific or educational organizations from engaging in politics. His proposal, also contained in the GOP platform, was a sop to Jerry Falwell Jr., the evangelical Christian president of Liberty University in Lynchburg, Va., who has endorsed Mr. Trump and long opposed the rule. Mr. Falwell and like-minded critics argue that it infringes the free-speech rights of clergy and their congregations — and that the Internal Revenue Service selectively enforces it against right-leaning Christians such as himself.

This is a can of worms better left unopened. To be sure, the law and accompanying IRS regulations are not models of legal clarity. They bar churches from "directly or indirectly participating in, or intervening in, any political campaign on behalf of (or in opposition to) any candidate for elective public office," upon pain of losing tax-exempt status. Turning over the Sunday collection plate to, say, the Clinton campaign, would fall on the wrong side of the line. But what about more common activities — publishing voter issue guides, railing about various campaign topics from the pulpit or, of particular relevance to historically black congregations, encouraging people to register to vote or turn out at the polls? Well, says the IRS, "certain activities or expenditures may not be prohibited depending on the facts and circumstances." Unavoidably, this puts government in the business of evaluating speech's content and is bound to have some chilling effect on someone.

Yet historical experience with the law is notable mainly for how little church-based political engagement it has prevented: Precious few organizations have even been investigated by the IRS; fewer still have been penalized. Indeed, more than 2,000 mainly evangelical Christian clergy have deliberately violated the law since 2008 as a form of protest against it; only one has been audited by the IRS, and none punished, according to the Alliance Defending Freedom, which supports these "Pulpit Freedom Sundays."

The best argument for the law’s constitutionality is that churches (and other nonprofits) themselves willingly accept its constraints in return for a very lucrative benefit: exemption from taxation. Those religious organizations committed to a form of witness that requires political activism are free to pursue it entirely on their own dimes. Conversely, do we really want a situation in which millions of dollars in political contributions could flow tax-free through any ostensibly nonprofit organization, from evangelical megachurches to — who knows? — Harvard University?

Awkward as it may be to live with the Johnson Amendment, it would be far worse, for the legitimacy of the political process, and the legitimacy of nonprofits themselves, to get rid of it. Not even a 2013 report by a commission of the Evangelical Council for Financial Accountability recommended repeal, though it did criticize the law and recommended reforms. The best alternative is to keep the Johnson Amendment, enforced discreetly and with as much evenhandedness as possible. That is to say: more or less the status quo.

Read more:

Richard W. Garnett: Tax exemptions protect religious freedom. We should keep them.

David Niose: Americans are leaving religion. Why are we still subsidizing it?

John Inazu: Want a vibrant public square? Support religious tax exemptions.

Miranda Fleischer: Churches are more private club than public good. Why do they need tax exemptions?

Jerry Falwell Jr: Here’s the backstory of why I endorsed Donald Trump