Todd Graham is director of debate at Southern Illinois University. His teams have won national championships for three years, and he's been recognized twice as the national debate coach of the year. The opinions expressed in this commentary are his.

(CNN) At Thursday's GOP debate in Miami -- the 12th in this campaign cycle -- I saw three serious contenders for the presidency. Why only three? Because the man that will most likely be the Republican nominee did not register: He continued to lack the thoughtfulness usually seen in a serious contender.

How did they all do as debaters? Let's grade them.

Ted Cruz: A-

His presence was excellent; his arguments on Social Security, trade and tariffs were sound. His description of how he plans to fix the economy and deal with taxes were all specific. I thought he was presidential. "Mean Ted" was gone, and "Statesman Ted" was in the house for a solid debate.

John Kasich: B

He mostly repeated his responses from the last three debates, but without the clarity of thought (his answers rambled) and without the energy. However, he still gets the B for breaking from the pack and defending science on climate change, arguing that we can have it both ways: cleaner energy and a strong economy.

Marco Rubio: B

JUST WATCHED Marco Rubio: 'The climate is always changing' Replay More Videos ... MUST WATCH Marco Rubio: 'The climate is always changing' 01:30

Rubio attacked less this time, which was a much stronger debate stance. And he added to that reasons he'd be a good President. His answers on Islam, veterans and Cuba were sound. But you can't earn an A if you fail, as he did, to admit that humans have a key role in climate change.

Donald Trump: F

Where to begin.

I know a lot of people will give Trump credit for not mudslinging. They'll heap praise on him for not interrupting and not name-calling. How pitiful is it that it's come to this? You know what? I don't give points for debate performances that meet the minimum for civility. Trump may have played nice this time, but that's not the same as debate excellence, especially with the mean, rude behavior of previous face-offs so close in memory.

In fact, Trump's policy weaknesses were on display in this debate, since without the name-calling, all that was left was for him to explain his positions.

JUST WATCHED Donald Trump, Marco Rubio spar over Cuba Replay More Videos ... MUST WATCH Donald Trump, Marco Rubio spar over Cuba 02:35

As it turns out, he only had two: negotiate better and make better deals. Wait, that's the same, so he only had one position. This fools no debate judge. It comes off as if Trump thinks negotiation, by itself, will solve the nation's problems, and that he is the only person who's ever thought of it. And if simply negotiating is enough, then why can't Trump tell us how he would negotiate, and for what? Cruz did. Rubio did. Kasich did. But not Trump.

Saying over and over that you'll do something is not persuasive in a debate.

How would Trump reverse Obama's policy with Cuba? A "better deal." How would Trump make China pay potentially economy-damaging tariffs? His high tariffs would only be used as a threat (I'm assuming for negotiations), and he offered no proof that this would work.

How would Trump solve the Israel-Palestinian dispute? "I'm a negotiator." Unfortunately for Trump, he then contradicted himself, saying that he would announce that he was "pro-Israel... I would like to at least have the other side think I am at least somewhat neutral to them" to get a deal done. Wait, what? These debates are televised, right?

JUST WATCHED Ted Cruz: Trump hand-raising pledge 'exactly backwards' Replay More Videos ... MUST WATCH Ted Cruz: Trump hand-raising pledge 'exactly backwards' 00:59

Trump was incoherent in many of his answers. When asked how he'd save Social Security, since, as debate moderator Dana Bash pointed out, cutting waste and fraud won't be nearly enough to close a funding gap, Trump responded with nonsense answers about a stronger military, Japan, North Korea, and China. How this answered the problem of Social Security for seniors was beyond reason.

When asked what was wrong with Common Core state standards in education, he blamed Washington D.C. and said politicians weren't interested in taking care of our children. But it's states which developed the Common Core and manage their own curricula. Trump didn't seem to understand it.

When quizzed about why he called Vladimir Putin "strong," and also China's leaders "strong" when they put down the Tiananmen Square protests, Trump said "strong does not mean good." Since he often characterizes his own approach as strong, it was another outright contradiction.

JUST WATCHED Donald Trump on rally violence: It's not me Replay More Videos ... MUST WATCH Donald Trump on rally violence: It's not me 01:45

Two even more frightening things came from Trump in the debate. Questioned by CNN's Jake Tapper about his earlier comment that "Islam hates the United States" he made the weak claim: "I mean a lot of them." This was rebutted by everyone else on stage.

And language matters. Tapper raised the question of whether Trump's rhetoric at his rallies inspires violence, such as the attack this week on a protester, who was punched in the face by a Trump supporter. Trump's very poor debate strategy was to lie and claim that protesters are some, "bad dudes, swinging, doing damage to people..."

You are failing at a debate when you bring on the bull.