On Sunday, Sen. Marco Rubio (R-Fla.) did something Sen. Rand Paul (R-Ky.) already did and Sen. Ted Cruz (R-Tex.) won't.

(Jonathan Ernst/Reuters) (Jonathan Ernst/Reuters)

We're talking about endorsing the reelection bid of the Senate's top ranking Republican, Mitch McConnell (Ky.), who faces a primary challenge from businessman Matt Bevin.

Rubio's decision was another reminder that (1) Cruz has zero interest in building bridges with the GOP establishment, and (2) his position goes against the grain of Paul and the Florida Republican, the Senate's two other highest-profile conservative members.

Rubio joined Paul, who has been in McConnell's corner for months. But Cruz won't be making it a trifecta of conservative Senate supporters. He said earlier this year that he intends to stay out of incumbent primaries, a decision which at the time was seen as a kick in the teeth of home-state Sen. John Cornyn (R-Tex.), who may also face a conservative challenge.

Paul clearly sees some practical benefit in endorsing McConnell. By offering McConnell a much needed dose of tea party support and the potential to bring his own backers with him, Paul is doing McConnell a solid favor.  And in politics, favors beget more favors.

Paul has signaled that if he runs for president in 2016, he won't do it the way his father did, by mostly shunning the GOP establishment. Instead, he is expected to cultivate relationships with Republicans like McConnell, whose help making inroads with establishment donors could be very, very useful.

Rubio clearly isn't shunning the establishment, either. He worked on crafting an immigration reform bill earlier this year, something the establishment wing of the party has been clamoring for in the wake of a disappointing election in which Republicans performed poorly among Hispanics. And he didn't hesitate to back McConnell, even as the Kentuckian faces an opponent backed by a national conservative group, the Senate Conservatives Fund.

But Cruz is charting an entirely different path. He's noted that he didn't come to the Senate to make friends. And he's not going to back any of his colleagues in primaries, an even more stinging posture considering that he is vice chairman of the National Republican Senatorial Committee, a group that is dedicated to incumbent protection.

What's more, during the government shutdown standoff, Cruz led the charge to defund the Affordable Care Act, further alienating himself from many leading Republicans. Rubio and Paul were very much in the background during that period.

Rubio, Paul and Cruz all arrived in the Senate in similar ways. Each of them defeated higher-profile Republican primary opponents. Where did their paths diverge? When they got to the Senate. Paul and Rubio clearly see value in playing nice. Cruz clearly does not.