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North Carolina voters are about to decide who they believe can best challenge Democratic Sen. Kay Hagan in November: a state politics insider backed by Mitt Romney or first-time candidates more aligned with the tea party and the Christian right.

Tuesday's U.S. Senate primary will highlight the struggle for control in the Republican Party as eight candidates run for the GOP nomination in a campaign that could influence the party and the makeup of the U.S. Senate. Republicans and independents can vote in the Republican primary. Hagan was being challenged in her own party by two lesser-known opponents.

Thom Tillis, the state House speaker and leading fundraiser in the GOP primary, is hoping to avoid a mid-July runoff by securing more than 40 percent of the vote.

"I'm not the one who believes it's certain," Tillis said on election eve. "I believe it's going to depend on turnout."

His leading rivals, obstetrician and tea party favorite Greg Brannon and Baptist pastor Mark Harris, want to block Tillis from winning, either now or in the summer.

"We believe that there's more to come," said Harris, whose backers include former Arkansas Gov. Mike Huckabee. "There's a strong possibility or probability of a runoff, and we've realized that we've got to keep going."

As Harris worked with volunteers knocking on doors Monday, Tillis and Brannon tried to use their endorsements to their advantage. Tillis rolled out his written endorsement from Romney, the 2012 Republican presidential nominee, who called Tillis a "conservative" and a problem-solver in state government. "I am confident he will do the same in Washington," Romney wrote in an email to Tillis' supporters.

Brannon, meanwhile, benefited from a visit to Charlotte by Kentucky Sen. Rand Paul, another tea party favorite.

North Carolina needs "a dragon slayer, and that dragon slayer is Dr. Greg Brannon," Paul told 250 people downtown near the NASCAR Hall of Fame, suggesting Brannon's strong conservative beliefs would shake up the status quo in Washington. "I'm here today because Greg Brannon is a true believer and we need true believers in Congress," Paul added.

The election-eve push was all about inspiring votes in Tuesday's GOP primary in a state that narrowly chose President Barack Obama in 2008 and Romney four years later. Tuesday's balloting is being hotly monitored in a year in which Republicans are six seats away from a Senate majority and determined to put electable candidates on the ballot.

Even Hagan, one of the Democrats' most vulnerable incumbents, got involved in the Republican primary by taking a page out of her party's political playbook. In a mailing, she hit Tillis, the fundraising leader in the GOP pack, for saying Obama's controversial health care law is "a great idea," even as he campaigns to repeal it. Tillis' full quote called the law "a great idea that can't be paid for." Hagan voted for the Affordable Care Act.

A PAC for Senate Democrats, meanwhile, has aired a TV ad about the severance packages given to two members of Tillis' legislative staff said to have had inappropriate personal relationships with lobbyists.

Brannon and Harris also have suggested Tillis could be unelectable in a general campaign given his political baggage. Tillis is probably the candidate for a voter "if you want an establishment ... style of United States senator, someone that is going to work in the system," Harris said.

Party leaders were hoping to settle their contest and unite as soon as possible. But it may be hard for some conservatives to join up with the ultimate GOP winner if the person isn't to their liking, especially Tillis.

"I'm not going to vote for somebody just because the party tells me to," said Lynn Davis, 58, a retired paralegal who attended Monday's Brannon-Paul rally in Charlotte.