But it's not such a mystery, really. Moderate congressional Republicans, after all — especially those in the Senate — want to hold onto their jobs.
Republican stalwarts that include Senate minority leader Mitch McConnell are nervous of losing their seats to social conservatives and Tea Party darlings in primary challenges ahead of next year's mid-term elections. As a result, they're stepping even further to the right to appeal to those socially conservative voters who turn out to vote in primaries.
McConnell has been aggressively fending off primary threats from Tea Party adherents for months in his home state of Kentucky.
Alison Lundergan Grimes, Kentucky's secretary of state, had been derided as "not ready for prime time" and a "left-wing mime" by McConnell loyalists. A pro-McConnell super-PAC said recently it was spending $270,000 on attack ads against Grimes.
In recent weeks, another Tea Party disciple has been making his move against McConnell, who has reportedly amassed a US$10 million war chest. Matt Bevin, a businessman who owns a 181-year-old bell-making company, is poised to announce his candidacy on Wednesday before heading out on a three-day tour of Kentucky.
"Matt has been speaking with grassroots activists throughout Kentucky and has received a great amount of encouragement and support," one of Bevin's advisers told Politico on Monday.
"This will be a real campaign, and we are ready for the inevitable smear tactics that Mitch McConnell's campaign machine is famous for. In the words of Kirsten Dunst, bring it on."
It all has the potential to become the type of migraine-inducing scenario that Republican brass was intent on avoiding next year. The party's elite watched in dismay in 2012 as Tea Party challengers successfully bested some established lawmakers in state primaries, only to go on to commit gaffes so grievous that Democrats snatched away seats that were previously Republican strongholds.
The most high-profile casualty was Dick Lugar, a six-term incumbent senator who went down to defeat to Richard Mourdock. In turn, Mourdock — who had repeatedly referred to his foe as "President Obama's favourite Republican" — was trounced by Democrat Joe Donnelly after asserting that babies resulting from rape were "a gift from God."
Other successful Republican primary challengers who went on to lose to Democrats were Todd Akin in Missouri, Sharron Angle in Nevada and Christine O'Donnell in Delaware.
This time, a primary challenger with a famous last name has emerged. Liz Cheney, daughter of former vice-president Dick Cheney, is taking on Wyoming Sen. Mike Enzi, a 69-year-old three-term lawmaker.
"I respect Sen. Enzi's 18 years of service, but it's time for a change and it's time for leadership," Cheney, 49, said last week. "It’s time for somebody who will be able to mobilize others."
Neither challenge to McConnell or Enzi will necessarily allow a Democrat to seize hold of those jurisdictions given they have long been in Republican hands.
But in several other states, including Iowa, Georgia and Alaska, there are fears of losing the seat entirely as a result of a challenge to an incumbent. Republicans need to gain six seats to win control the Senate.
There are also concerns among the party brass not just about the internal bad blood that flows from such primary brawls, but how a coerced step to the right by candidates with mainstream appeal will hurt Republican chances of not only of securing a majority in the U.S. Senate, but of winning back the White House in 2016.
Sen. Marco Rubio, for example, is now laying low on his immigration reform mission amid displeasure from the Tea Party — even though he isn't facing re-election until '16.
Rubio is, however, eyeing a run for president that year. In recent weeks, he's been vowing to vote against any spending bill that doesn't defund Obamacare, is considering becoming the lead sponsor of an anti-abortion bill and has been hitting Obama hard for his "job-killing" environmental agenda — all music to the Tea Party's ears.
On Tuesday on Capitol Hill, Rubio will face several fellow Republicans no longer enamoured of the Florida lawmaker. Enzi has also been invited to the Senate Tea Party caucus meeting, expected to be attended by dozens of Republican legislators.
"I have heard repeatedly from people in Florida that they are ready to look for primary challengers, and I have heard from people around this entire country that they don't want him to be the presidential nominee in 2016," Jenny Beth Martin, co-founder of the Tea Party Patriots, said in a recent interview.
Nonetheless the group organizing Tuesday's meeting, TheTeaParty.Net, said it had arranged the gathering in the spirit of "open arms."