There's no shortage of Republicans looking to unseat senators in primaries this year. But quantity is outpacing quality so far.

Kentucky candidate Matt Bevin (R). (Mike Lawrence/The Gleaner via AP )

Six of the 12 Republican senators running for reelection face primary challengers. But most of the contenders have yet to make serious inroads against the incumbents they are trying to knock off. Some have raised scant campaign cash. Others have been dogged by questions about their past. And none have built the groundswell of support necessary to do something as difficult knocking off a sitting senator before the general election.

It's early in the cycle, but as it stands now, the field of challengers is wider than it is deep.

The highest-profile race is in Kentucky, where Senate Minority Leader Mitch McConnell faces businessman Matt Bevin. McConnell's not a popular figure in Kentucky, but Bevin has yet to seize on his image problems to narrow the gap. He raised less than half the money McConnell brought in during the final three months of last year and polls show him lagging behind badly. Meanwhile, Bevin's own record has come under scrutiny. He's railed against the TARP program, yet he backed it as an investment fund president in 2008. Bevin has about three months until the election to reverse the tide.

Challengers to Sens. Lindsey O. Graham (S.C.) and Lamar Alexander (Tenn.) have fizzled. Graham's challengers lag behind him badly in the money chase and Alexander's main opponent trails him by a wide margin according to a recent poll conducted for the senator's campaign. In Texas, Rep. Steve Stockman has been more distracting story than a serious opponent to Sen. John Cornyn.

It's not easy to dislodge sitting senators in primaries. It's only happened eight times in the past 30 years, according to a tally from the University of Virginia's Center for Politics.

But half those instances came during the past two election cycles, amid the rise of the tea party. Some senators like Richard Lugar (R-Ind.) in 2012 and Robert Bennett (R-Utah) in 2010 were unprepared for the onslaught and backlash, and lost as a result. GOP Strategists say that the current crop of senators facing reelection learned from those mistakes, preparing early by raising money and reaching out to conservative activists and groups.

In Texas, Cornyn recruited a tea party organizer to manage his campaign. McConnell enlisted the support of Sen. Rand Paul, his junior colleague and a tea party rock star, early in his effort.

"Almost without exception Senators heeded the lessons from the last two cycles and have taken nothing for granted. They built solid campaign war chests, assembled top-flight campaign teams, and are criss-crossing their states," said Brian Walsh, a former National Republican Senatorial Committee spokesman.

The challenger who appears to have the best chance right now is Chris McDaniel, a Mississippi state senator running against Sen. Thad Cochran. McDaniel has won the support of the Club For Growth and the Senate Conservatives Fund, and he's proven that he can raise money. Cochran was rated the 34th most conservative senator in 2013, according to National Journal's vote ratings, making him one of the more moderate members of the GOP Conference.

McDaniel said technological advances have allowed challengers to compete more heavily with long-established incumbents.

"People are beginning to peer behind the veil for the first time in an awful long time," said McDaniel. "They're scared and they're anxious, so they've begun to focus on these votes. And they have the ability now to do it with the Internet and social media, whereas 10 years ago, many people didn't have that opportunity."

But McDaniel appears the be the exception to the rule this cycle. And even as he's impressed conservative activists early, he far from a shoo-in to win.

Conservative strategists and candidates openly acknowledge the challenge they face this cycle. But they say they are not displeased with the landscape.

"Every challenger starts out as a long shot, so it's actually pretty remarkable that so many of them are competitive this year. If you told me a year ago that Mitch McConnell, Thad Cochran and Pat Roberts would all be fighting for their political lives today, I would not have believed it," said Senate Conservatives Fund executive director Matt Hoskins.

It tends to take missteps from the incumbent to open the door for a challenger. Lugar, for example, badly bungled questions about his residency. Sen. Pat Roberts (Kan.) could fall victim to the same kind of scenario this year, if he doesn't put similar questions behind him.

The New York Times reported last week that Roberts rents out the home he owns in Kansas and is registered to vote at the address of supporters whom he pays rent to stay with when he is in town. Roberts's opponent Milton Wolf, a doctor and second cousin once removed to President Obama, released a radio ad slamming Roberts on the matter Wednesday.

It's far too early in the cycle to declare these races sealed up for the incumbents. Richard Mourdock, for example, trailed Lugar for much of the race in 2012 before closing in down the stretch.

But the Republican senators are in decent shape, especially considering as conservative challengers have been flooding the zone.