Known as ACORN, it runs one of the largest community voter registration drives in the nation, signing up more than 1.3 million mostly minority and low-income people this year alone.
But now top Republicans are calling for a "full-scale" investigation of their operation, and McCain is calling the organization out, CBS News chief investigative correspondent Armen Keteyian reports.
"There are serious allegations of voter fraud," McCain said this week.
Joseph Borges, the registrar of voters in Bridgeport, Conn., says his office has been overwhelmed by bogus forms filled out with thousands of fake and duplicate names - two boxes alone packed with non-existent addresses - all submitted by ACORN.
"There's something wrong with the process, to have this happen for a registration drive," said Borges.
Since June, election officials in at least a dozen states have raised red flags over more than 10,000 voter registration forms filed by ACORN. For example, in one Ohio county ACORN registered the same person 17 times.
And earlier this week law enforcement agents raided ACORN's office in Las Vegas, carting away files and computers.
"It was purely politically motivated," said Bertha Lewis, a chief organizer for ACORN. "The right wing and specifically the Republicans."
Lewis is the chief organizer for ACORN. She claims only a tiny fraction of their new registrations have problems. Problems, critics charge, related to the pressure ACORN reportedly puts on its workers to up their numbers - at any cost.
"Do you have quotas that people have to fill every day?" Keteyian asked Lewis.
"No," she said. "We pay people by the hour. And also, we train folks on a daily basis."
But CBS News has talked to nearly two dozen ACORN workers who all said they were pressured to increase their number of registration cards - to the point of copying names out of phone books, signing up inmates, and registering the dead.
"They just have to maintain 25 cards a day," Borges said. "That's a quota, as far as I'm concerned."
Whether simple error or outright fraud, the charges surrounding ACORN are already raising doubts about the integrity of the upcoming election in key parts of the country.