By John Whitesides
WASHINGTON (Reuters) - Republicans in North Carolina will vote on Tuesday on their candidate for a U.S. Senate seat in November, in the first of several primaries this month that will test whether the party's establishment can beat back challenges from Tea Party rivals.
State House Speaker Thom Tillis, backed by party mainstream leaders and business groups, appears to have an edge over Tea Party favorite Greg Brannon and evangelical minister Mark Harris in North Carolina, but he needs 40 percent of the vote to avoid a costly July runoff with the second-place finisher.
The showdown to determine who will take on vulnerable Democratic Senator Kay Hagan kicks off a month-long string of Republican primaries that could prove instrumental to the party's efforts to retake control of the Senate in November.
Republican leaders are anxious to limit the kind of divisive primary fights that produced weak general election candidates and helped cost them winnable Senate seats in 2010 and 2012.
Establishment-backed Republican Senate candidates also lead opinion polls in two May 20 contests: in Kentucky, where Republican Senate leader Mitch McConnell is favored over a Tea Party challenger, and in Georgia, where a crowded primary makes a runoff likely but the most conservative Tea Party candidates are not among the poll leaders.
Republican campaign officials also hope for a primary win on May 20 in Oregon by first-time candidate Monica Wehby, a pediatric neurosurgeon with the sort of moderate image that might be successful in a Democratic-leaning state.
This year, Republicans in Congress have learned to be more proactive against challenges, analysts said, shifting to the right and courting conservatives while raising more early money and going on the attack when necessary.
"What we have seen this cycle is that every Republican incumbent took a potential primary challenge seriously and was ready for it early on," said Republican strategist Brian Walsh, a consultant and former aide at the National Republican Senatorial Committee.
Some party voters also have suffered "buyer's remorse," he said, after watching candidates like Todd Akin in Missouri and Richard Mourdock in Indiana ride the momentum of the Tea Party movement to primary wins, then flame out in the general election with campaign-trail missteps such as Akin's 2012 comment about "legitimate rape."
This year, Republicans need to gain six seats for a Senate majority and cannot afford to fumble away chances at a time when President Barack Obama's low approval ratings and public dissatisfaction with his healthcare law have Democrats on the defensive.
'COMMON SENSE CONSERVATIVE'
In North Carolina, Tillis has emphasized his credentials as a social and fiscal conservative to counter his rivals in a contest that also has attracted attention from a trio of potential 2016 presidential candidates.
Republican Senator Rand Paul of Kentucky campaigned for Brannon on Monday, while Harris is backed by former Arkansas Governor Mike Huckabee and former Florida Governor Jeb Bush has endorsed Tillis. Brannon and Harris have questioned Tillis's conservatism, but polls show Tillis with a solid lead and support near the 40-percent level needed to avoid a runoff.
He has been helped by two powerful advocacy groups linked with the party's establishment - the business-friendly Chamber of Commerce and American Crossroads, founded by strategist Karl Rove - which together spent about $2.6 million on ads for him.
A Crossroads ad aired in Charlotte and Raleigh describes Tillis as a "common sense conservative" and highlights his support for tax cuts and his opposition to the healthcare law, while linking Hagan to Obama and describing her as addicted to "out of control" spending and debt.
Steven Law, president of Crossroads, said the group was looking for Republicans with the strongest shot at winning in November. It also has advertised in Alaska for former state attorney general Ray Sullivan, in a contested Senate primary for the right to face endangered Democratic Senator Mark Begich.
"We definitely took away from 2010 and 2012 a concern about opportunities lost because of poor candidate performance," Law said. "Our view is that Tillis is far and away the most competitive candidate to challenge Hagan."
In other North Carolina contests, former "American Idol" TV show runner-up Clay Aiken is battling former state Commerce Secretary Keith Crisco for the Democratic nomination to challenge Republican Representative Renee Ellmers, and Republican Representative Walter Jones faces political consultant Tyler Griffin in a primary.
Indiana and Ohio also hold nominating contests on Tuesday as the U.S. election season gets under way in earnest. Ohio Democrats are expected to choose Ed FitzGerald, a former FBI agent and now Cuyahoga County executive, over minor opposition to take on Republican Governor John Kasich in what will be one of the top governor's races in November.
(Editing by Alistair Bell and Mohammad Zargham)