Let's zero in on a telling moment in Tuesday night's Republican presidential debate.

A little more than halfway in, Sen. Rand Paul (R-Ky.) and Sen. Marco Rubio (R-Fla.) got into a heated back-and-forth about how much to spend on America's military, with Rubio accusing Paul of being an isolationist and Paul accusing Rubio of wanting to spend too much on defense and bankrupting the country.

The exchange was so contentious that other candidates wanted to get involved, lest a key moment pass by without their input. Many of them tried to interject all at once.

Sen. Ted Cruz (R-Tex.) jumped in to say, hey, there's a middle ground between the two.

Donald Trump cut in: "We have to make our military bigger, better and stronger than ever before."

Carly Fiorina moved to jump in, and John Kasich also attempted to have his voice heard before the moderator cut them all off and moved on.

You notice whose name we didn't mention yet? Jeb Bush, who stayed silent throughout the whole thing. In fact, the only other candidate who didn't attempt to get involved was Ben Carson, who is leading in the polls and didn't really need to.

The former Florida governor, if you believe analysts like The Fix, very much needs to inject more ! into his campaign. After a half-hearted, ham-handed performance two weeks ago on the debate stage and with slipping poll numbers, the one-time front-runner needed a spark to show he has a pulse in the GOP primary.

The problem is, Bush proved yet again in Tuesday's GOP debate he's just not that kind of guy. Whatever strengths he has as a politician, debating simply isn't among them. At all.

[VIDEO: Why Rubio's win over Bush at the CNBC debate is so detrimental]

He started out the debate earlier trying to be more assertive. Cruz had just finished delivering his plan to get the economy going, and the moderator, Maria Bartiromo, wanted to go to Bush when Kasich tried to interject.

Bush, perhaps remembering the advice of a media coach he recently hired for debate prep, decided to try to shut Kasich down.

"You've already made two comments, John. It's my turn," he said.

The moderator assured Kasich she'd be back to him soon.

Bush (perhaps not-so-wisely bringing up his performance in the last debate) tried to get Kasich to stay quiet, saying, "I got about four minutes in the last debate. I'm going to get my question right now."

And yet, after all of that, Kasich started talking -- as politicians are wont to do -- and Bush inexplicably let him go.

"I appreciate it, Jeb," Kasich almost patronizingly said. "I'm all for ya. But I want at some point to talk about a value-added tax and $11 trillion, $12 trillion tax cuts that will put our kids way deeper in the hole than they have been at this point. So I would like to talk about it at some point, because that's what leadership is."

Bush soon got to talk, but not before Bartiromo asked a new question. It was, as Washington Post reporter David Weigel points out, classic Bush.

The third time attempting to command the stage for himself was not the charm for Bush, either. Almost 20 minutes after staying mum during the Rand-Rubio military spending drama, he decided again to try to chime in.

But this time he was attempting to interrupt the master of interruptions: Donald Trump.

Once again, Bush did it half-heartedly, interjecting a word or two and hoping Trump would miraculously stop talking about his plan to protect America from the Islamic State and give Bush his turn. (In a barb that went straight to the heart of Bush's debate problem, Trump had joked earlier in the debate; "Give Jeb Bush his turn.")

Trump apparently didn't follow his own advice.

He steamrolled through Bush's meek interjections, at one point holding up his hands like a man used to getting his way to indicate he's still talking. Sure enough, Bush seemed to give it to him. He fell quiet.

Bush waited until Trump had finished speaking entirely to make his point. But when he finally did, the emotion had been sucked out of the room by Trump.

His delivery didn't help things either:

"Donald is absolutely wrong on this." he said, pointing his finger like a college professor. "That's like a board game. That's like playing Monopoly or something. That's not how the real world works."

Bush received hardly any applause -- nor was there time for it -- before Trump jumped back in and gave a much-more animated reply. He got the last word. And the difference in animation was clear as day.

The audience clapped and soon forgot about Bush, who once again faded into the background in a debate where he really needed to do something more.