A watchdog group has asked federal regulators to close a “loophole” former 2020 presidential candidate Michael Bloomberg used to donate $18 million to the Democratic National Committee (DNC) last month.

Citizens United petitioned the Federal Election Commission (FEC) on Wednesday to close the “Bloomberg loophole” that, in effect, allows self-funded political candidates to give money to certain political groups in excess of individual limits placed on donations. The activist group said that the Bloomberg campaign’s donation, almost entirely derived from Bloomberg’s personal wealth, was “more than 500 times greater” than what he could have directly given.

“While Bloomberg’s transfer may fall within the letter of the regulation governing transfers of candidate funds to national political party committees it certainly does not fall within the spirit,” wrote Michael Boos, executive vice president and general counsel of Citizens United.

Federal regulations cap individual donations to the DNC at $355,000 per year. Boos noted that Bloomberg “was at least a legitimate candidate” who appeared on ballots in a handful of states and spent around $900 million before dropping out of the Democratic primary, the most expensive political campaign ever in American politics.

“But there is nothing in the current regulatory framework that would prevent many candidates – whether serious or not – from manipulating the loophole to an even greater degree,” Boos continued. “Wealthy individuals could: declare their candidacy for any federal elected office; contribute untold millions of dollars of his or her own money to the campaign; promptly withdraw his or her candidacy after spending a token sum; and thereafter transfer the balance of the campaign’s funds to the national party committee of his or her choice.”

Boos said that the law could be patched by linking a campaign’s maximum allowable donation to a committee or national political party to the number of people that donated to the campaign.

Last week, Americans for Public Trust filed a petition with the FEC alleging that the Bloomberg campaign’s donation did violate the law and that Bloomberg should be held accountable for “circumvent[ing] federal contribution limits,” according to The Washington Post. The pro-Trump Great America PAC has also asked the elections oversight agency to investigate Bloomberg.

The former New York City mayor dropped out of the Democratic primary on March 4 after a being trounced on Super Tuesday despite spending hundreds of millions on an aggressive advertising campaign. Bloomberg managed to win 45 delegates, spending roughly $12.2 million per before exiting.

Bloomberg’s exit surprised many, especially his staffers, after his continued promised to stay in the race until the convention. The wealthy New Yorker also allegedly promised his staff that, no matter the outcome, he would continue to pay them through the November election. Bloomberg laid off thousands after dropping out and providing the DNC with $18 million from his campaign treasury.

Bloomberg’s fired campaign staff sued the billionaire in late March alleging that thousands of former staffers were promised a steady paycheck for the rest of the election cycle. Bloomberg later attempted to soften the blow of unemployment to his former staffers by gifting them their work laptops and iPhones.