The year-long Miami-Dade state attorney’s office investigation into Congressman David Rivera’s personal and campaign finances has stalled over questions about more than $100,000 in undisclosed campaign donations that appear to fall under a little-known loophole in Florida’s campaign-finance laws.

Rivera collected the money not for a political office but for a campaign he mounted for an obscure, unpaid position within the Republican Party. Campaigns for party posts are not governed by state election laws, allowing Rivera to raise and spend as much money as he wanted without public disclosure or contribution limits.

That exception could make it difficult for investigators to charge Rivera with any wrongdoing, if the money they have been probing turns out to be unregulated by state law.

State prosecutors, working with the Florida Department of Law Enforcement, discovered the funds while probing Rivera’s finances dating back to his years in the state House, including payments from a Miami gambling company to a firm with ties to Rivera.

For months, investigators have been examining how Rivera raised the money and how he spent it, according to sources close to the probe. Prosecutors are also trying to determine if Rivera’s use of the money may have run afoul of other laws beyond the campaign-finance statutes, the sources said.

Rivera, a first-term congressman, is also the subject of a second criminal probe by the FBI and the Internal Revenue Service. The two agencies are examining possible tax-evasion questions stemming from $510,000 in undisclosed payments from the former Flagler Dog Track — now known as the Magic City Casino — to Millennium Marketing, a company owned by Rivera’s mother and godmother.

Plagued by the investigations, Rivera, a once-impressive fundraiser, has struggled to bring in money for his congressional re-election bid. His opponent, Democratic state Rep. Luis Garcia, outraised him by nearly four to one in the quarter that ended in October. Rivera’s district spans from western Miami-Dade to eastern Collier County.

Rivera’s campaign responded with a two-sentence statement to a list of questions e-mailed by The Miami Herald, which worked on the story with news partner WFOR-CBS 4.

“Any money raised and spent in relation to Congressman Rivera’s campaigns for the Republican Party of Florida state executive committee was done with the strictest adherence to all applicable legal requirements and any suggestion to the contrary is completely false,” the statement said. “Congressman Rivera has at all times acted in compliance with both the letter and spirit of campaign finance regulations and has timely, properly and publicly reported all personal income.”

The criminal probes have focused on Rivera’s finances dating back to 2006, when the Miami lawmaker was climbing up the Florida House leadership ranks. At the same time, however, Rivera held another role: Miami-Dade Republican Executive Committeeman, a position to which he was first elected in 2004.

In Florida, Republican voters elect a committeeman and a committeewoman from each county to make decisions on the statewide executive board, recruit candidates, fundraise and influence the direction of the party. (Democrats appoint their committee members.) The GOP races rarely draw much attention or involve any campaigning.