Tonight's North Carolina Primary Is The Latest Skirmish Between The GOP Establishment And The Grassroots

Forty will be the magic number in the latest big Republican showdown Tuesday.

It's the percentage of the vote North Carolina House Speaker Thom Tillis needs to hit to avoid a runoff in the most significant GOP primary of the 2014 election cycle thus far. Tillis' main challenger is obstetrician Greg Brannon. The two are battling to face off against an incumbent considered to be one of the most vulnerable Democrats in the country, Sen. Kay Hagan.

Any path toward a GOP Senate majority likely has to include taking out Hagan in North Carolina. But Republicans' hopes of a Senate takeover isn't the only thing hanging in the balance in this race. The election is shaping up to be the latest proxy battle in the Republican Party, with Tillis the choice of the so-called GOP "establishment" and Brannon backed by the Tea Party and libertarian types that make up the conservative grassroots.

Tillis has been endorsed by both former Republican presidential candidate Mitt Romney, and potential 2016 Republican presidential candidate Jeb Bush. He is the preferred candidate of the National Republican Senatorial Commitee. And he has received more than $10 million in support from establishment-aligned groups, including the Chamber of Commerce, which recently went up with an ad buy in the state.

"This is a critical primary race for the U.S. Chamber, and we have been heavily involved to support Thom Tillis, who believes in free enterprise. It is vitally important that we have leaders in Washington who will prioritize growth and opportunity," a Chamber spokesman said in an email.

Brannon, meanwhile, has the support of libertarian and Tea Party-approved Kentucky Sen. Rand Paul, who made a last-minute stop to campaign with him in the state on Monday. He's also endorsed by the grassroots group FreedomWorks.

Establishment insiders worry about the possibility of a July runoff for two reasons. First, there's a legitimate chance Tillis could lose in a two-way fight with Brannon, who has gained momentum over the past week. A Public Policy Polling survey released Monday found Tillis just on the cusp of the 40% threshold.

The poll also found that, in the event of a runoff, Tillis would start out leading Brannon only 46-40. That's a decidedly narrow margin, particularly since history has shown runoffs favor the candidate perceived as more conservative since more engaged voters are likely to turn out. The most recent example of this phenomenon was seen in Texas in 2012, when then-candidate Ted Cruz rode a wave of momentum to a July runoff victory.

And if Tillis does prevail in a runoff, it would put him in a bad position to take on Hagan with an additional two-month grind that would likely push him rightward. Dragging into a runoff would also expose Tillis to a potential political minefield, since the North Carolina legislature reconvenes in May.

"Those are two things that are a difficult balance for him," said Tom Jensen, the director of Public Policy Polling, which is based in Raleigh. "Because a lot of stuff that happens in the legislature that he doesn't necessarily have any control over - with the caucus getting a little more extreme and saying crazier stuff. That's something that would be an unwelcome distraction for him in a runoff campaign."

The Hagan campaign has done its best to take advantage of the discord in the GOP primary. Over the past week, it has been behind mailers and radio ads that note Tillis once referred to the Affordable Care Act as a "great idea." Tillis' campaign responded by accusing Team Hagan of taking him out of context, noting he said in his fuller quote it's a "great idea that can't be paid for."

Some observers have described Hagan's campaign as taking a page from the playbooks of Senate Majority Leader Harry Reid and Missouri Sen. Claire McCaskill, both of whom meddled in their opponents' primaries in order to face a much weaker opponent on their way to general-election victories.

However, one source close to Team Hagan told Business Insider the campaign doesn't really have a preferred outcome on Tuesday. On one hand, a runoff would be beneficial with the two Republican candidates slugging it out for another two months. On the other, it could also benefit Hagan to have a clear opponent it could start to define as soon as possible.

The source said the ads blasting Tillis on Obamacare are the start of that contrast. It forced Tillis to clarify his position where the Hagan campaign said he was trying to "have it both ways." Indeed, Tillis recently emphasized that he supports fully repealing the federal health-care law.

"Nobody would shed any tears if Republicans got pushed into a runoff," the Hagan source said. "But we have a big, bright contrast to draw in a state that is darn-near 50-50 at the presidential level. We've got the resources to do it. So if the general election starts tomorrow, well, that's fine with us."