The scene was set for two former political buddies to become adversaries Wednesday night. Only one was truly ready for the fight.

Sen. Marco Rubio (R-Fla.) walked into CNBC's Republican primary debate under fire from allies of his former mentor, former Florida governor Jeb Bush, for missing votes in the Senate.

A Bush-supporting super PAC, "Right to Rise," had set up a Twitter account earlier Wednesday, @IsMarcoWorking, mocking Rubio's absence in Washington. And sure enough, once the debate got started, Bush went for the attack.

Well, kind of.

"When you signed up for this, this is a six-year term, and you should be showing up to work," Bush said, turning to his former mentee in the Florida legislature, who was standing right next to him by virtue of where the polls stand right now. "You can campaign or just resign and let someone else take the job."

This has been in the news for days, and Rubio was clearly ready for it. Squaring his shoulders and widening his stance, he immediately blasted back that Bush was making something out of nothing.

Bush is modeling his campaign comeback off 2008 nominee Sen. John McCain, is he not, Rubio asked with lawyerly precision. Well, McCain missed a lot of votes, Rubio went on, his voice rising over Bush's half-hearted objections.

"I don't remember you ever complaining about John McCain's vote record," Rubio said.

He then added: "The only reason why you're doing it now is because we're running for the same position, and someone convinced you that attacking me is going to help you."

Mic drop.

As is made clear in the video above, the difference between the two men is that one half-heartedly, a bit awkwardly and clearly uncomfortably jumped into a fight he wasn't prepared for. The other dove in fearlessly and got the last word.

It was a memorable moment for both men, but not for the reasons Bush, who is lagging in the polls and clearly sees Rubio as a threat to his GOP establishment support, would want.

It was a telling scene from Bush, who just this weekend lamented the fact that he didn't much like having to demonize his political opponents. That was clearly the case when it comes to going after someone with whom he has been close. His heart wasn't in it, and you can't attack someone halfway -- as Tim Pawlenty 2011 could attest.

Pawlenty, much like Bush, was a would-be front-runner who kind-of-sort-of attacked the other establishment favorite in the race, Mitt Romney, for his Obamacare-esque health-care plan as governor of Massachusetts. At a debate, though, Pawlenty would not follow through on it. It was symbolic and symptomatic of his undoing.

Bush's failure to win the point was also costly in the sense that this debate was an important one for Bush, who needed to reassure his donors that he's got what it takes in this debate. That he lost it to Rubio should be particularly troubling.

Bush then didn't help himself a few moments later when he was asked if he would raise taxes $1 if he got $10 in tax cuts -- a by-now-standard debate question. Bush replied if a Democrat came to him with that proposal, he would give them "a warm kiss."

Again, awkward. Nothing that will end Bush's campaign Wednesday night, mind you. But it furthers the existing narrative about Bush's campaign -- that he just can't win the day-to-day campaign battles and isn't a great messenger or campaigner.

[Nobody has more on the line at Wednesday's debate than Jeb Bush]

But then again -- perhaps debate skills stand in the eye of the beholder. Bush's campaign was even playing up the attack, despite real-time social media reactions going overwhelmingly against them.