The Republican race for an open U.S. Senate seat has so far been defined by the familiar ground of outsiders vs. insiders and a fight over which contender is more conservative than the next. But a schism emerged at a forum Monday over a tax debate that is one of the sharpest policy differences yet.

Six of the leading GOP hopefuls each vowed to reject calls to extend unemployment insurance and vote against a comprehensive immigration overhaul gelling in the Senate. Yet the rift erupted over a controversial Internet sales tax bill, and that’s noteworthy in a race largely defined by candidates who take the same positions on nearly every issue.

It involves pending legislation that would require Internet retailers to collect sales tax for state and local governments, and debate over the measure has divided tea party types and establishment Republicans. The Senate has already passed the measure, but anti-tax groups in the House have vowed to keep it from a vote.

U.S. Reps. Phil Gingrey, Jack Kingston and Paul Broun, along with former Georgia Secretary of State Karen Handel, all voiced opposition to the measure, rejecting it as a tax increase that should only be considered as part of a broader overhaul.

Kingston reinforced the view after the forum, saying “I do not support tax increases for the Internet. I do believe we need a comprehensive overhaul.”

David Perdue, a former Dollar General chief executive, took a sharply different tack.

“Being the only retailer up here, I can tell you I’ve seen both sides,” said Perdue, eliciting loud applause from a crowd of local officials at the forum hosted by the Georgia Municipal Association. “The federal government should make sure all players are playing on an equal playing field. We should have the same rules apply to both brick-and-mortar and online retailers.”

It was a rare sign of dissonance in a race dominated by the candidates’ struggles to distinguish themselves ahead of record-early May 20 primaries. The winner can expect a well-financed candidacy by Democrats eager to flip the seat held by retiring Republican U.S. Sen. Saxby Chambliss. Michelle Nunn, a nonprofit executive and daughter of former U.S. Sen. Sam Nunn, is her party’s leading contender.

With no clear GOP front-runner, some candidates have targeted Kingston, the early fundraising leader, while others took aim at Broun, seen by some Republicans as too conservative to survive a November general election contest.

The latter approach played out after Broun defended his campaign's plan to give a free AR-15 to a supporter as a good idea by declaring Georgia a "pro-gun state." Gingrey responded by suggesting the gun giveaway was pure political pandering and said that every candidate on the stage was sufficiently pro-gun rights.

After the forum, Gingrey repeated the point in an interview with reporters.

“Paul is a good man. I’m not accusing Paul of invoking a gimmick,” he said, adding: “It’s a little gimmicky.”

Kingston, meanwhile, made clear he had his eye on November. The Savannah Republican used his closing to harangue Nunn for refusing to participate in a Democratic debate that was set to follow the GOP one. Nunn has said she would attend debates closer to the May vote.

“I’m appalled that the leading Democrat opponent has refused to come up before you and answer the questions,” Kingston said.

In her stead, Democratic contender Branko Radulovacki seized the opportunity. The psychiatrist, better known as Dr. Rad, told a dwindling crowd he was spoiling for a fight.

“I’m all fired up,” he said. “I don’t know why we have to wait another six months. I can take on all of them right now.”