PROVIDENCE, R.I. — Secretary of State Matt Brown has been running for the U.S. Senate on a clean-government platform, but he now finds himself dogged by questions about donations his campaign received from Democratic parties in other states.

Brown received $25,000 from state Democratic parties in Hawai'i, Maine and Massachusetts days before four of his top donors gave $30,000 to those parties. The donors already had given Brown the maximum they could under the law.

Critics accuse Brown's campaign of arranging illegal donations, and on Wednesday, the Hawai'i Republican Party signed a complaint asking the Federal Elections Commission to investigate the matter. Brown has said he did nothing illegal, but he's returning the money because it created an "appearance problem."

The contributions have shed light on a practice that campaign finance experts say is an increasingly common way of skirting federal campaign laws.

"This is a run-of-the mill campaign finance tactic," said Nathaniel Persily, a University of Pennsylvania Law School professor who specializes in election laws.

Brown, Rhode Island's top elections official, is one of two Democrats running for the seat held by Republican Sen. Lincoln Chafee. Chafee, a moderate in a heavily Democratic state, faces a conservative challenger in the primary.

The race is considered one of the best chances for Democrats to win a seat in the Republican-dominated Senate and has already become one of the most expensive in the nation. The four candidates have spent almost $3 million with the primary still six months away, according to the Center for Responsive Politics, a nonpartisan group that tracks campaign finances.

Brown has said that late last year, his campaign field director contacted the parties, asked them to contribute and offered to help them raise money in return. Brown describes the offer as a well-intentioned effort to help Democrats around the country.

But the treasurer of the Hawai'i Democratic Party told The Associated Press earlier this month that the campaign and party struck a deal in which the party gave money to Brown in exchange for money from Brown supporters. She later said there was no deal.

Earmarking money for a specific candidate and funneling it through a third party to avoid the $4,200 limit on individual donations to campaigns is illegal under federal law, campaign finance experts say.

Many people legally give to multiple candidates or to both candidates and political parties.

It becomes illegal when donors give to a party — or other organization — with instructions that the money be sent to a particular candidate, said Larry Noble, executive director of Center for Responsive Politics and a former general counsel for the Federal Elections Commission.

"There's a fine line between it being just a suggestion that you'll help fundraise and it becoming an earmarked contribution," Noble said.