The complaint focuses on the Donald J. Trump Foundation, a nonprofit founded by Trump in the late 1980s to give away profits from his book, "The Art of the Deal." Today, Trump remains the foundation's president, but the charity's money now comes from other donors: Tax records show no gifts from Trump himself to his namesake foundation since 2008.
The foundation is quite small, by the standards of others listed as billionaires. The Trump Foundation has no full-time staff, and gave away just $591,000 in 2014 — the last year for which records are available.
Early this year, the Trump Foundation wound up at the center of a controversy that briefly engulfed Trump's campaign.
It began on Jan. 28 in Iowa, on a night that Fox News was hosting a GOP primary debate. Trump — still feuding with Fox News anchor Megyn Kelly — skipped the event, and held his own televised fundraiser for veterans' causes.
That night, Trump raised about $4.5 million from other donors, and about $2 million of it passed through the Trump Foundation. In the days afterward, Trump handed out some of the money at campaign rallies in Iowa and New Hampshire, calling up local veterans' groups and handing them an oversized check.
On the check, often, were the words "Donald J. Trump Foundation" and "Make America Great Again," the slogan of the Trump presidential campaign.
In its complaint, the watchdog group asserts this was a violation of IRS rules governing nonprofits. Those rules state that nonprofits like the Trump Foundation are "absolutely prohibited from directly or indirectly participating in, or intervening in, any political campaign."
"In the context of a campaign rally designed to advocate for Mr. Trump's nomination ... the use of the slogan [by the Trump Foundation] can have no reasonable meaning other than to urge his election," the watchdog group's complaint says.
If the IRS did decide to investigate the Trump Foundation, it could — in theory — strip away its tax-exempt status. But experts on IRS procedures said that was unlikely. In addition, they said, any IRS investigation was not likely to start until the foundation files its returns for 2016, well after the presidential election is over.
The Trump campaign, which has answered inquiries about the Trump Foundation in the past, did not immediately respond to a request for comment sent Monday morning.
The Trump Foundation has already been the subject of two previous complaints by liberal watchdog groups.
In 2013, for instance, the Trump Foundation made a donation to a political group affiliated with Florida Attorney General Pam Bondi (R). That seemed to violate the IRS's rules about nonprofits becoming involved in politics. Then — because of what Trump officials called a clerical error — the foundation did not notify the IRS.
Instead, the Trump Foundation told the IRS that this donation had actually gone to an unrelated group in Kansas with a name very similar to that of the political group in Florida. The effect of the error was to cover up a prohibited kind of gift, and make it appear to be a permissible one.
Another complaint, by the American Democracy Legal Fund, focused on news first reported in The Washington Post. In 2012, Trump used funds from the foundation to buy a football helmet autographed by quarterback Tim Tebow. Tax experts said that, if Trump kept the helmet for himself, he would have violated another section of the IRS rules -- which prohibit foundation leaders from using a charity's money to buy things for themselves.
Trump has not said what he did with the helmet. The IRS has not commented on any of these complaints.