Churches generally use the donations they collect from their congregations to cover basic operating expenses—things like paying church staff, building maintenance, and community outreach. But if the new Republican tax bill passes, your weekly offering could be used to pay for airing a political attack ad.

Tucked into the 429-page tax overhaul bill is a provision that would repeal the Johnson Amendment, a section of federal tax law, dating back to 1954, that is supposed to prevent churches and other tax-exempt religious organizations from engaging in politics. Since churches are tax-exempt organizations, the idea is that taxpayers shouldn’t be forced to subsidize groups that are advocating for candidates and ballot items that they may not agree with.

If the Johnson Amendment is repealed, churches will be free to endorse political candidates, as well as raise and spend unlimited amounts of money on politicians’ behalf. And because churches are considered 501(c)(3) non-profits, they wouldn’t have to disclose the identity of their donors.

This would create a new loophole making it easier than ever for anyone—special interest groups, billionaire oligarchs, foreign governments, etc.—to secretly finance politicians.

Unlike just about every other kind of non-profit, churches don’t have to file 990s with the IRS. Even political dark money 501(c)(4) non-profits like Crossroads GPS and Patriot Majority USA have to file 990s that make some information about their fundraising and spending publicly available (albeit not with enough granularity to reliably trace the money back to its source). Without 990s, churches could give political donors more anonymity than any other type of organization, and following that money would be nearly impossible.

Furthermore, unlike every other form of political giving, political donations to churches and religious groups would be tax-deductible. That means that in addition to the unparalleled level of secrecy they would get, political donors would have a major financial incentive to funnel their contributions through religious groups.

Americans don’t want churches engaging in politics.

According to public polling data, 71 percent of Americans want to keep the Johnson Amendment intact to prevent churches from engaging in political campaigning. Even a majority of white evangelical protestants, a demographic that voted overwhelmingly for Trump, oppose exempting churches from the Johnson Amendment, the polling found.

So why is Congress pursuing this?

Big money has more influence in the government than ever before, and Congress would be doing major donors a huge favor by creating a new tax-deductible loophole they could use to secretly funnel money into politics. The billionaires that play an increasingly large role in funding elections could directly benefit from a repeal of the Johnson Amendment, and the politicians that make that possible could see payback in the form of funds for their reelection.