In recent days, similar claims against Mr. Sproul have arisen in Nevada and Colorado.

Mr. Sproul, 40, a former executive director of the Arizona Christian Coalition and the Republican Party in Arizona, is well known in political circles there. Since 2004, Mr. Sproul’s companies — he has operated under several corporate names — have collected more than $17.6 million from Republican committees, candidates and the “super PAC” American Crossroads, mostly for voter registration operations, according to campaign finance records.

The Republican Party, which paid Mr. Sproul about $3 million this year for work in five states, has severed its ties with him, saying it has no tolerance for voter registration fraud.

But questions about Mr. Sproul’s methods first emerged in 2004, when one of his companies, Sproul & Associates, was paid nearly $8 million during the election cycle. The payouts made the company the seventh-biggest recipient of campaign expenditures by the committee, according to an analysis by the Center for Responsive Politics.

Mr. Sproul declined to be interviewed.

In a statement issued by his lawyer, Mr. Sproul said the huge size of his voter operation — he claims to have registered more than 500,000 people in more than 40 states through election cycles — would invariably lead to a few problems. “Inevitably, there have been accusations of ‘bad registrations,’ isolated instances that have been thoroughly investigated not only internally but by the appropriate legal authorities,” the statement said.

Mike Hellon, a former chairman of the Arizona Republican Party, said that Mr. Sproul had been considered “very controversial” in Arizona Republican circles before the recent allegations, partly because of past voter registration investigations. “There are questions among a lot of people in the party about how he gets these contracts and why he gets contracts,” Mr. Hellon said.