(CNN) On Monday night, sources close to Donald Trump promised that he hadn't finished his feud with Tennessee Sen. Bob Corker. They weren't kidding!

"The Failing @nytimes set Liddle' Bob Corker up by recording his conversation," Trump tweeted Tuesday morning . "Was made to sound a fool, and that's what I am dealing with!"

Corker is relatively short in stature -- he's 5'7" -- while Trump is right around 6'3". Corker's height was, according to the Daily Caller , one of the big reasons the Tennessee Republican didn't make the cut to be Trump's secretary of state.

"Two sources with knowledge of the transition say that the Tennessee senator's short stature was a key factor," the Daily Caller report said. Yes, really.

The focus on Corker's height is the latest example of the fact that the President of the United States name-calls like your average fifth-grade bully. As he demonstrated throughout the 2016 campaign, this is Trump's default approach to politics -- and life. He has a unique gift to zero in on people's weaknesses and then pick at them relentlessly.

Lyin' Ted. Lil' Marco. Low Energy Jeb. Crooked Hillary. Little Rocket Man. Pocahontas.

It worked like a charm in the campaign. Trump's voters loved his lack of political correctness. They loved that he called politicians out. They loved that he refused to apologize for anything.

But the lowest and darkest moment came in South Carolina in November 2015, when Trump mocked New York Times reporter Serge Kovaleski after he pointed out the candidate's assertions about "thousands" of people celebrating the 9/11 attacks in the New York area were false.

"Now, the poor guy, you ought to see this guy, 'Ah, I don't know what I said, I don't remember, I don't remember, maybe that's what I said,'" Trump said, wildly flailing his arms. That gesturing was widely interpreted as Trump mocking Kovaleski, who has arthrogryposis, a congenital condition that creates arm movements very similar to those Trump used to describe the reporter. (Trump denied he was mocking Kovaleski. You be the judge .)

Trump's voters seemed to overlook or condone his bullying under the theory that (a) he wasn't really serious, (b) his status as a change agent mattered more than his personal character or (c) they kind-of liked it because they hated the people he was bullying.

There was also some sense -- among his supporters and even those who didn't vote for him and couldn't even imagine him becoming president -- that once the gravity of the office hit him, he might change. There was some thinking -- occasionally stoked by Trump himself -- that he was only acting like this because it was how he had to win the campaign. Once he was elected president, everything would change.

"I will be so presidential, you will be so bored," Trump promised in April 2016 . "You'll say, 'Can't he have a little more energy?'"

Trump didn't change. If anything, he has become more of a name-calling bully than he was in the campaign. (Side bar: How many 71-year-old men fundamentally change their personality? If you answered "none," you got it right!)

He got in a fight with the Australian prime minister.

He shoved aside -- literally! -- the prime minister of Montenegro.

He's taunted North Korean dictator Kim Jong Un.

I could go on (and on) but you get the idea. Bullies bully. Name callers call names. It's what they do.

There are two impacts of the fact that the President is a bully. (You can like him or hate him, but it's hard to see how you can conclude that he isn't one at this point.)

The first is that Trump's almost-obsessive need to always respond any time he feels as though he has been attacked or his character has been questioned makes for a terrible legislative strategy.

Including his ongoing fight with Corker, Trump has insulted or personally impugned well more than a dozen Republican senators. That's stunning given that Republicans only have a two-seat majority in the Senate and, as the health care fight showed, have almost no margin for error on anything but the most benign pieces of legislation.

Trump is cutting off his nose to spite his face with these attacks. On tax reform, Iran, immigration and North Korea (and any other issue you can name) there is no path to success for Trump that doesn't include Corker and lots of the other senators he has attacked.

The second -- and more important -- point here is that Trump, whether he likes it or not, is a role model. People look to our leaders for guidelines as to how they should act. When the President acts like a bully -- even with people he needs to work with -- he gives significant license for others to follow his lead. He normalizes this sort of behavior. He makes it OK for people to treat one another the way he seems to treat, well, everyone.

That's a very bad thing -- whether you are a Democrat, a Republican or don't care about politics at all.