The man at the center of a voter-fraud investigation in Florida is a longtime GOP operative with a pugnacious streak and a controversial history among Republicans and Democrats for his aggressive tactics in registering voters.

Nathan Sproul is a businessman who rose rapidly in Arizona politics, heading the local Christian Coalition and the state Republican Party in his mid-20s. A series of voter-fraud allegations against him in 2004 did not slow his ascent: Sproul and firms linked to him have been paid $21.2 million by the Republican Party, its candidates and affiliated interest groups over the past nine years.

But the man who once accused his Democratic critics of having “hysterical fits” now finds himself isolated politically. Nine Florida counties reported to the state that hundreds of voter-registration forms submitted by Sproul’s firm contained irregularities such as suspicious signatures and missing information. State law enforcement authorities are investigating the allegations. A spokesman for Sproul denied any improprieties, saying Sproul’s firm “has never tolerated even minimal violations of election law when registering voters.’’

Late last week, the party that Sproul has worked so hard to build severed ties with him. GOP officials in Florida, who had paid Sproul’s Strategic Allied Consulting $1.3 million this election cycle to help register and turn out voters, filed an election complaint against his firm with state officials.

Sproul and firms linked to him also were paid $1.6 million from state parties in North Carolina, Virginia and Colorado, and Mitt Romney’s campaign paid him $72,000 for “field consulting” in this election.

Nathan Sproul, right, former executive director of Arizona's Republican Party and the state's Christian Coalition branch, poses for a photo in his Phoenix office, Monday, Oct. 25, 2004. (AP Photo/Tom Hood) (TOM HOOD/ASSOCIATED PRESS)

“We take the integrity of elections extremely seriously,’’ Sean Spicer, a spokesman for the Republican National Committee, said Monday. “We have zero tolerance for even the mere allegation of impropriety.”

Voter registration is an especially sensitive issue this year as a series of restrictive voter-access laws have become a flash point in the presidential campaign. President Obama’s supporters say the measures target minorities and other pro-Obama groups. Republicans say they are needed to combat voter fraud, and it was a law passed last year in Florida that enabled the probe of Sproul to move forward: It requires groups that register voters to put their organization’s names on every application they submit.

On Monday, Democrats on the House Committee on Oversight and Government Reform requested an interview with Sproul about the allegations.

Sproul, 40, declined to be interviewed by The Washington Post but a spokesman, David Leibowitz, said in an e-mailed statement that Sproul’s firm is cooperating with election authorities and “will continue to do everything within our power to uncover any unethical or illegal activity in Florida.”

“Obviously, everyone at [Sproul’s firm] is disappointed by the end of a years-long fruitful relationship” with the Republican Party, Leibowitz added, “but they understand why this was done. There can be no distractions right now.”

Sproul, who has operated a network of at least five Arizona-based consulting firms since 2003, said Republican National Committee officials asked him to establish a new firm to shield the party from earlier allegations against him, according to the South Florida Sun-Sentinel. Strategic Allied Consulting was incorporated in June in Virginia. Sproul’s name is not listed on corporate documents.

Spicer said he is “unaware that that ever occurred.”

Sproul grew up in Arizona Republican politics. He moved to Tempe at age 2 and graduated from Pillsbury Baptist Bible College in Minnesota in 1994, according to a 2004 Associated Press profile of him. After briefly interning in Washington for then-Rep. Jon Kyl (R-Ariz.), he returned to Arizona, where he headed the Christian Coalition and, starting in 1999, the state Republican Party.

“It was pretty obvious he was talented,” said Randy Pullen, the Arizona GOP chairman from 2007 to 2011, who praised Sproul for his hard-charging tactics and effectiveness. “You can’t say that he’s milquetoast.”

Several Arizona Republicans said Sproul is known for his zeal in bringing voters into the Republican camp.

“Nathan was one of those who was willing to do or say anything possible to get a job and get the job done,” said Sean McCaffrey, who took over as state party executive director in 2007. He said he kept files of reports of complaints in the media or to the state party against Sproul and said he would not support party candidates who worked with Sproul.

In 2004, former canvassers for Sproul’s firm came forward in four states alleging that they were told to register only Republicans, with some saying that registration forms completed by Democrats were thrown out. The Justice Department investigated the allegations but did not bring charges, according to congressional testimony.

At the time, Sproul accused Democrats of “having hysterical fits about how well we did our job” and showing their own “lack of integrity” by feigning anger when “they do the exact same thing every day of the week.”

In the statement Monday, Sproul’s spokesman, Leibowitz, called the 2004 allegations “isolated instances” and said the company was cleared of any wrongdoing.

Alice Crites contributed to this report.