is far from certain. Kentucky Sen. Rand Paul, a possible White House candidate with substantial Tea Party clout, is headed to the Tar Heel state Monday to stump for one candidate, libertarian-leaning Greg Brannon. Another Republican contender, Charlotte pastor Mark Harris, has been endorsed by former Republican presidential candidate Mike Huckabee.

In November, Republicans stand their best chance in years of winning the Senate majority and controlling the agenda on Capitol Hill during the final years of President Obama's second term. Republicans need to net six seats to take control of the chamber.

Republicans tied to the party's leaders want to avoid a repeat of the 2010 and 2012 elections in which Tea Party-aligned Senate candidates won key Republican primaries in states such as Delaware, Nevada, Colorado, Indiana and Missourionly to lose to Democrats in the general election.

"Our goal was to make sure that we had the highest quality crop of candidates possible to compete in the fall elections," said Steven Law, the CEO of American Crossroads, a super PAC tied to Republican strategist Karl Rove. The group also been active in Alaska's three-way GOP primary.

After 2012's stinging defeats, the Crossroads pledged to intervene in primary fights to thwart far-right conservatives likely to falter in general-election battles. Aided by prominent Republican donors, the group collected $5.2 million in March to help in primary fights — more than it raised in all of 2013.

Last week, the U.S. Chamber of Commerce announced ad buys to help pro-business candidates in six states with upcoming primaries.

"The establishment groups are fighting back