Sen. Marco Rubio of Florida won’t take sides in GOP incumbent primaries because of his own experience of running against the establishment’s pick. Neither will Sen. Mike Lee of Utah, who rode tea party support to take down a three-term incumbent. Sens. Rand Paul of Kentucky and Ted Cruz of Texas are also unlikely to back any of the conservatives taking on Republican senators; in fact, Paul is committing heresy in the eyes of tea-party hardliners by endorsing two Washington insiders, Kentucky Sen. Mitch McConnell and Wyoming Sen. Mike Enzi.

This show of decorum from senators who instigated the unpopular government shutdown is striking at a time of mounting friction between the establishment and tea-party wings of the Republican Party. So what’s behind it? The upshot of the tea party caucus largely staying on the sidelines — and in Paul’s case, endorsing two of his colleagues — is that of all the protocols the conservative insurgency has trashed on Capitol Hill, a member endorsing a colleague’s opponent remains strictly taboo.

“It’s a club, and once they are part of the club and learn the secret handshake, they all look out for each other,” said Matt Hoskins, a former Capitol Hill staffer who serves as executive director of the Senate Conservatives Fund.