— As 2013 closed out, Republican U.S. Senate candidates seemed mostly focused on finding different ways to hammer Democratic U.S. Sen. Kay Hagan and her support of the Affordable Care Act.

But before the new year dawned, Cary obstetrician Dr. Greg Brannon slammed state House Speaker Thom Tillis as "an establishment-backed insider" in a fundraising appeal. Among other things, Brannon cited a scandal that forced two Tillis staffers to resign.

"His Chief of Staff – who shared an apartment with my opponent – and his Policy Analyst were forced to resign after it was revealed they were having 'inappropriate' relationships with lobbyists," Brannon wrote in the Dec. 31 fundraising appeal. "But my opponent still gave both of them 'golden parachutes' -- a full month's pay after they resigned from their position."

It was only a matter of time before Republican primary candidates turned their rhetoric on one another, said North Carolina State University political science professor Andrew Taylor.

"Both political parties are much more homogeneous than they were 30 or 40 years ago," Taylor said. "So, when you're in a primary, you need to show some differences somewhere."

Along with Rev. Mark Harris, past president of the Baptist State Convention of North Carolina, Tillis and Brannon are the front runners in the GOP field, which also includes Greensboro radio talker Bill Flynn and Wilkesboro nurse Heather Grant. Tillis has gotten the backing of established political figures, such as Karl Rove, a former aide to President George W. Bush, while Brannon and Harris have generated the most buzz among the grassroots tea party wing of the Republican Party.

Over the early part of the campaign, Tillis, Brannon and Harris have displayed few differences on the big issues. For example, on the Affordable Care Act, all three say "Obamacare" ought to be repealed.

While Tillis isn't a household name statewide, Taylor said, Brannon and Harris still have further to go in fundraising and gathering adherents.

"Particularly for candidates like Harris and Brannon, you need to be able to change that dynamic and get some earned media coverage however you can," Taylor said.

For his part, Tillis is also positioning himself as the front-runner in the campaign, airing a television ad that continues to focus on Hagan and refusing to respond to Brannon's provocation.

"My only response is we're focused on beating Kay Hagan – period," said Tillis spokesman Jordan Shaw. "That's what the Republican voters and conservative voters in this state are focused on."

As in other states, North Carolina's Republican primary candidates can be expected to sort themselves into "establishment" versus "tea party" camps, Taylor said.

Tillis, as the sitting state House speaker, is the only member of the current GOP primary field to be serving as an elected official. Although, as Nathan Gonzales writing for Roll Call notes, Tillis doesn't mention that fact in his campaign ad.

Taylor said Tillis' status as a top-ranking lawmaker may not be all positive, as Brannon's email shows. Opponents can use issues that cropped up during his time leading the House against him during the campaign.

Brannon's email plays on people's frustrations with politicians of all stripes and attempts to make Tillis responsible not just for his own foibles but the missteps of an entire political class.

"This particular election cycle isn't unique," Taylor said. "There are many other cases where experience in government is a liability rather than an asset."

Brannon has worked hard to claim the tea party mantel and has hired two senior staffers that previously worked for Republican Sens. Rand Paul of Kentucky and Ted Cruz of Texas, both tea party favorites.

"The race is currently a primary contest for the Republican nomination," said Reilly O'Neal, Brannon's campaign manager. "We will show a clear contrast between our opponent, who is backed by establishment insiders like Karl Rove, and a servant citizen, backed by grassroots conservatives, who will actually fight for our party's conservative values in the U.S. Senate."

Harris doesn't quite fit in the tea party versus establishment dynamic, but he is developing his bona fides as an emissary from the party's social conservative wing. He has touted his ties to former GOP presidential candidate Mike Huckabee and recently landed an endorsement from the Concerned Women Political Action Committee, which praised him as "a pro-life, pro-family conservative," lauding his work for an amendment banning gay marriage in North Carolina.

He can also lay claim to some veteran political backing. Mike Rusher, his campaign manager, is a former chief of staff for the state Republican Party Robin Hayes, the former state party chairman, also backs his campaign.