The Argentine and Venezuelan governments deny involvement
A Uruguayan man has pleaded guilty in a Miami court to taking part in an alleged Venezuelan plot to smuggle $800,000 (£400,000) into Argentina.
Rodolfo Wanseele, the third person to admit involvement, pleaded guilty to working with Venezuelan intelligence.
US prosecutors say the money was intended to finance the election campaign of Argentine President Cristina Fernandez last year.
Both the Venezuelan and Argentine governments deny any involvement.
The alleged plot first came to light when a Venezuelan-American businessman, Guido Antonini Wilson, was stopped as he tried to enter Argentina with the money in a suitcase last August.
US prosecutors say Mr Antonini was not aware of the contents of the suitcase, is not accused of any crime and that he is co-operating with US police.
However, Argentina is seeking his extradition from the US for questioning.
As part of a plea bargain, Wanseele, 40, pleaded guilty to acting as an illegal foreign agent in the US.
He admitted a relatively minor role in the alleged plot.
He drove a suspected Venezuelan intelligence agent, Antonio Jose Canchica Gomez, from Miami airport to a meeting with Mr Antonini at a Starbucks restaurant in Florida.
Wanseele faces a possible 10-year jail sentence, though the term is expected to be shorter because he has co-operated with the investigation.
Canchica, 37, is accused by US prosecutors of acting for Venezuelan President Hugo Chavez and trying to get Mr Antonini to cover up the origins of the money. Canchica remains at large.
What became known as the "suitcase scandal" sparked allegations of corruption, and increased diplomatic tensions between Argentina, Venezuela and the US.
At the time, Ms Fernandez described the accusations as a "trashing operation", while Venezuela rejected them as a "fabricated scandal".
But Assistant US Attorney Thomas Mulvihill has said that an FBI recording shows that the money was destined for Ms Fernandez's campaign.
The alleged leader of the group, Venezuelan Franklin Duran, 40, intends to plead not guilty to involvement at a trial scheduled for September, according to his attorney.
Two others have pleaded guilty. Venezuelan Carlos Kauffmann, 35, admitted in March to acting as an unregistered agent of a foreign government and agreed to co-operate with prosecutors. He is expected to be sentenced in May.
Another Venezuelan, Moises Maionica, 36, entered a guilty plea in January.