As bullies go, Donald Trump is unusually skilled.

When Trump decides to go after you, he considers carefully both your weak points and the audience for his attack. So when he decided to pummel Jeb Bush — apparently for his own amusement, as much as out of any real political concerns — he hit upon the idea that Bush was "low energy," something Bush had a hard time countering without sounding like a whiny grade-schooler saying, "Am not!" More than anything else it was a dominance display, a way of showing voters he could push Jeb around and there was nothing Jeb could do about it. With a primary electorate primed by years of watching their candidates fetishize manliness and aggression, the attack touched a nerve.

And now with the Republican race effectively narrowed to three candidates, the one Trump hasn't bothered to go after too often — Marco Rubio — must prepare for the mockery and rumor-mongering that will surely be coming his way from the frontrunner. Whether he can withstand it could go a long way toward determining how this race turns out.

Until now, Trump has been relatively soft on Rubio. But with the increasing possibility that Rubio could be the greatest threat to Trump winning the nomination, he's almost certain to go after him. If the past is any guide, Trump will throw a bunch of different attacks Rubio's way until he happens upon one that seems to resonate; then he'll stick with it as long as it works. Trump is already dabbling in Rubio birtherism (though he doesn't seem quite committed to it), but eventually he'll find a line of personal criticism with just the right note of cruelty and derision.

Rubio already seems spooked. Appearing on Face the Nation this Sunday, he was asked how he would convince voters to choose him over Trump, and the strongest critique he could muster was that Trump hasn't been clear enough about his policy plans. But Rubio went out of his way to assure everyone he wasn't being mean. "So, look, this is not an attack or anything of that nature," he said. "It's just a very simple observation. If you want to be president, you have to start detailing some specific public policy." Yowch, put away the shiv, senator!

Rubio may have avoided Trump's wrath up until now, but that won't last. The only question is what brand of contempt Trump will heap on him. It might be some kind of attack based on Rubio's ethnicity, or it might be the same kind of you're-a-girly-man insults he used on Bush. That could be effective, since Rubio does look like he didn't graduate high school all that long ago. He could go after Rubio's occasionally shaky finances, which Trump surely looks on with utter contempt, since as far as he's concerned, not being rich makes you a loser.

Or perhaps Trump will tell voters that Rubio isn't strong enough to channel their free-floating rage. Trump tweeted on Monday that he won the South Carolina primary because "I showed anger and the people of our country are very angry!" Whether anger fully accounts for that particular result, there's no doubt that it fuels much of Trump's popularity.

Up until now, Rubio hasn't been very good at expressing anger. When he does, it comes out awkwardly, like the endless repetition of "Barack Obama knows exactly what he's doing" that got him into so much trouble before the New Hampshire primary. He has gone back to being a candidate of optimism: "I will bring this party together faster than anyone else," Rubio now argues, which might be true. The trouble is that anger remains the predominant emotion running through the Republican electorate, and they don't particularly want to be brought together, if it means joining up with the establishment that now sees Rubio as its last hope of defeating Trump.

If Rubio ends up being his party's nominee, it will mean that Trump came after him and he survived the onslaught. Because Trump will indeed come after him. He'll bait him and belittle him, insult him and mock him, laugh at him and sneer at him. And it will be a test of Rubio's ability to stand up and fight back, like a real man. Rubio will have to figure out how to fend it off, because nobody else has.