Why have some religious conservatives decided to support Donald Trump for United States president? Leaders have named their reasons: He’s promised to appoint pro-life Supreme Court justices; he’s allegedly good at business. But they have also consistently cited something else, perhaps more unexpected: the tax code.

Trump has promised to repeal the so-called Johnson Amendment, a 1954 provision that prohibits tax-exempt organizations from participating in political activities. Proposed by then-Senator Lyndon B. Johnson and later revised by Congress, it keeps churches and other non-profits from lobbying for specific causes, campaigning on behalf of politicians, and supporting or opposing candidates for office.

While opponents of the Johnson Amendment often frame their objections in terms of free speech, the provision’s primary impact may be financial. Right now, the IRS makes a clear distinction between non-profit groups—from charities and universities to certain private schools and houses of worship—and political organizations.

If the Johnson Amendment were repealed, pastors would be able to endorse candidates from the pulpit, which they’re currently not allowed to do by law. But it’s also true that a lot more money could possibly flow into politics via donations to churches and other religious organizations. That could mean religious groups would become much more powerful political forces in American politics—and it would almost certainly tee up future court battles.

Even though religious groups are some of the most vocal opponents of the Amendment today, it was originally about something else: communism. At the time when the measure was passed, McCarthyism was at its peak, and Johnson feared that right-wing groups, parading as charities, would attack his reelection campaign. Although the rule extended to religious groups, the former Purdue University professor James D. Davidson has argued that Johnson never specifically wanted to target religious groups.

According to the Catholic University of America professor Roger Colinvaux, some critics have argued that the Amendment’s history is the best argument against it: Because it was an ad hoc measure written to satisfy one skilled legislator’s political needs, they say, it should be repealed. But, as Colinvaux wrote in 2012, this already was a long-standing issue by the time Johnson took it up—the legal limits around political activity for non-profit groups “had dogged charitable tax status from the inception of the federal-income-tax exemption for charitable organizations.”

Congress first approved a tax deduction for donations to charitable organizations in 1917, but the boundaries around those organizations’ political activities weren’t exactly clear. In a 1930 decision, Slee v. Commissioner of Internal Revenue, a Second Circuit Court judge made those boundaries clearer: He ruled that the government doesn’t have an obligation to subsidize the political activity of non-profit groups; and people can write off donations to these groups, which include religious organizations, but not if the groups are engaging in “political agitation,” or lobbying. In 1934, this rule officially became part of the tax code: “No substantial part of an organization’s activities” could involve “carrying on propaganda or otherwise attempting to influence legislation.”

There were two problems with this rule. First, the word “substantial” is vague and confusing. “People found that standard difficult to meet because they couldn’t identify it—they couldn’t quantify it,” said Miriam Galston, a law professor at George Washington University. “The IRS never gave any clear or precise guidelines.” Subsequent court decisions made this standard somewhat clearer: “Substantial” is somewhere between 5 and 20 percent of an organization’s operating budget and efforts, Galston said, and factors like mission and volunteer time have to be taken into account.

The other problem was that lobbying isn’t the same as electioneering—a non-profit group like a church might not spend time and money trying to get a bill passed in Congress, but it might promote a candidate for office with flyers and buttons and speeches. The Johnson Amendment clarified that the ban extended to political activity: Non-profits, including religious groups, couldn’t support candidates for political office without losing their tax-exempt status. In 1987, Congress clarified that this means non-profits can’t oppose candidates, either.

The IRS doesn’t often go after churches.

Since 2008, a group of predominantly conservative, Protestant churches have participated in Pulpit Freedom Sunday—a day started by the conservative legal group Alliance Defending Freedom, when hundreds of pastors across the country give explicitly political sermons in protest of the IRS’s rule. The movement has been growing, and religious leaders will often mail tapes of their sermons directly to the agency to showcase their defiance.

The IRS doesn’t often go after these churches, though. Agency leaders have emphasized the importance of educating religious organizations about what is and is not legal, rather than aggressively initiating audits or trying to revoke the non-profit status of houses of worship. The agency has rarely pursued this last option; one of the most prominent recent cases was in 1995, when it denied the non-profit status of an upstate New York church that took out a full-page ad in USA Today warning Christians not to vote for Bill Clinton in 1992.

In general, though, “the political climate has changed in the last four or five years, where attacks on the IRS has been more frequent, more virulent, and the IRS has become extremely defensive,” Galston said. Especially with the budget cuts of the last half decade, the agency has scarce resources for enforcement of the Internal Revenue Code. “They can’t, in my view, allocate the resources in such a way that they preclude proper enforcement of the other code sections,” Galston added.

Yet even beyond purposeful protests like Pulpit Freedom Sunday, religious leaders seem to openly defy the ban on participating in political activities. The televangelist Mark Burns has openly stumped for Trump, as has Liberty University president Jerry Falwell Jr. And at the start of the Democratic National Convention, the Decatur, Georgia, pastor Cynthia Hale prayed for Hillary Clinton to become president. Even if the IRS would not see these actions as formal violations of the law, the difference between pastors electioneering and speaking as private citizens “is a fine distinction that is easily evaded,” said Galston.

Critics of the agency, including some progressive religious groups, argue that the IRS should put more resources toward enforcing the electioneering ban. The main question, said Alan Brownstein, a law professor at the University of California, Davis, is not whether religious groups and leaders should be able to express their views—it’s whether that activity should be subsidized by the government. “Pastors can say whatever they want, as can anyone else,” he said. “The question is whether a tax-exempt institution can say whatever it wants and retain its tax-exempt status, and whether the pastor as an official can use his or her position in the tax-exempt institution to engage in electioneering.”

Although religious groups often participate in political and campaign activities in defiance of the law, only Congress could make these activities fully legal. And doing so would raise big questions about money: Would religious organizations get to keep their tax-exempt status if they were permitted to participate in campaigns and endorse candidates? More importantly, could people still make tax-deductible donations to religious organizations—effectively giving money to promote a particular candidate or campaign?

Congress could make the first change fairly easily: It could add language to the current provision adding a special exception for pastors and other religious leaders who want to talk politics from the pulpit, Galston said, although that might present constitutional challenges. It would be a little more complicated to legalize tax-deductible donations to politically active churches. As Galston pointed out, another provision of the tax code strictly bans charitable, tax-deductible contributions to organizations that engage in political activity. Moreover, all charitable organizations are forbidden from providing “private benefit” to any individual—which includes campaigning on their behalf. So Congress would have to amend those provisions as well.

It’s unlikely that a President Trump would be able to push this repeal through Congress.

The result would likely be two-fold: Social-welfare groups that currently file as 501(c)(4)s, for example, might apply to become 501(c)(3)s in order to get a better break on taxes, Galston said. Moreover, political donors might start directing more cash toward non-profits, since those donations would be tax deductible. If all of these changes were made—which seems fairly unlikely—the biggest beneficiaries would likely be the wealthy. Tax-deductible donations only benefit people who take itemized deductions; people with high incomes are significantly more likely to do so.

What’s unclear about Trump’s promise to repeal the Johnson Amendment, though, is whether he’s only intending to push a repeal of the rule for religious organizations. A broad change to the provision would likely cause minor-level chaos within the U.S. political system: There would no longer be any meaningful difference between charitable groups and lobbying organizations. The government would effectively be subsidizing the political activities of all schools, charities, churches, and scientific-research organizations. On the other hand, if Trump’s theoretical administration pushed for a repeal only for religious groups, legal challenges would almost certainly follow. “It would be a preference for religion against organizations that were not religious,” Galston said.

Realistically, it seems unlikely that a President Trump would be able to push this repeal through Congress—it would pose immense political and legal challenges for legislators, Republicans and Democrats alike. The proposal seems to serve more of a dog-whistle purpose during this 2016 presidential campaign season: It’s a signal to religious conservatives that Trump is their champion, and that he cares about religious-freedom issues. It might also be a message to rich conservatives, specifically: Here, Trump hints, might be a way to make tax-favored political donations.

For those Americans who want more, not less, religious influence on American politics, the repeal of the Johnson Amendment is the perfect campaign promise: a guarantee of increased political power, greater freedom of speech, and more control over political dollars for groups that widely feel their electoral influence slipping.