Editor’s Note: Todd Graham is the director of debate at Southern Illinois University. His debate teams have won five national championships and he has been recognized three times as the national debate coach of the year. Follow him on Facebook and Twitter. The views expressed in this commentary belong solely to the author. View more opinion at CNN.

(CNN) —  

The sixth Democratic debate started uneventfully with almost no daylight appearing between the candidates for over an hour. But in the debate’s second half, when the debaters finally warmed up and began to find areas to distinguish themselves, it was the women who came out on top in my gradebook.

Todd Graham
Todd Graham
PHOTO: Courtesy of Todd Graham

A-

Amy Klobuchar

Klobuchar had the best answer to the first question in the debate about impeachment. She touched on President Donald Trump “calling a foreign leader to look for dirt on a political opponent” and argued that he should allow his administration to testify and “not be scared to put forward his own witnesses” – important points in persuading more people that Trump should be impeached. Other Democrats on stage gave far more standard answers when explaining why they thought Trump was a bad president.

I teach my students that there’s a fine line between assertiveness, which is good, and aggression, which is often seen as abrasive. Klobuchar walks that fine line in almost every debate. Thursday night, she got the better of Mayor Pete Buttigieg with her direct, yet not rude, comments about his lack of experience and how the other Democratic contenders have accomplished much more than he has.

Klobuchar also walked the line between progressive and pragmatic when she said that “I just don’t think anyone has a monopoly on bold ideas. I think you can be progressive and practical at the same time.” I’m always asked who can stand up to Trump in a debate, and Klobuchar demonstrated last night that she’d have no problem handling Trump’s insults or abrasiveness. Klobuchar can go on the offensive with the best of the candidates.

B+

Elizabeth Warren

Warren and Buttigieg had a lively debate over campaign financing, with Warren pressing the mayor over his fundraising at a wine cave. I thought this was the best part of the debate, with both candidates making sound, reasonable arguments for their positions: Warren charged that Buttigieg was opening himself up to big money influence by holding exclusive fundraisers, and that we need to keep money out of politics, since the candidates receiving it are often beholden to the source after they are elected. She later was able to pivot that argument into her themes of income inequality, corporate greed, and corruption.

I very much liked Warren’s statement that she would, in the Rose Garden, annually read the names of transgender women and people of color who have been killed in the last year. It was touching. Still, she got lost in the debate at times, and didn’t appear to have as much conviction as I’ve seen her display in past debates.

B-

Andrew Yang

Yang stayed out of the main scrums, which helped and hurt the businessman. He was able to press his freedom dividend several times in the debate, which is one of his strengths. But there were moments when I wanted to hear more from Yang, and that means that he’s got to be a bit more assertive at times.

This debate featured fewer players, so it would have benefitted Yang to try to wrestle more speaking time. I liked his previous above-the-fray approach to an overly crowded debate stage, but found his trepidation last night overly cautious. Compare this with Klobuchar, who inserted herself into many different topics.

As usual, when Yang spoke, I thought he made excellent arguments, and his comment on how special needs children are the new normal likely hit home with a lot of viewers.

B-

Bernie Sanders

Sanders was able to pick at the edges of a couple big skirmishes, while still remaining fairly undamaged. When Warren and Buttigieg were going back and forth about big campaign donors, Bernie had a cute line about Mayor Pete needing to catch Joe Biden in billionaire donors, since he was trailing by a count of 44 to 39. Unfortunately, on many occasions, Sanders’ speaking delivery verged on yelling, a choice that simply doesn’t suit him well. His last two debates were his best, and it’s mostly because he remained calm. I’d like that Bernie back.

C

Pete Buttigieg

Buttigieg had his worst debate yet. When attacked, it’s important to keep your cool. Mayor Pete handled his first foray against Elizabeth Warren well. While the optics of fundraising at a wine cave are about as bad as you can get, Mayor Pete held his ground. Buttigieg’s retort to Warren was sound: to beat Trump, he argued, the Democrats should use all means at their disposal – including money from wealthy donors.

However, after that, Amy Klobuchar seemed to get under his skin when she began a series of attacks about the value of experience. His responses were incongruously sharp in tone, yet unfocused in content. Buttigieg’s answer about his relationship to the First Amendment was both unclear in its relevancy and out of sync with the debate. Pro tip: If you ever have to say “And before the break…” as Buttigieg did in this instance, just let it go. Nobody can remember what the point was in the first place, but by picking back up a lost cause, it reminds everyone that you are still steaming about something that happened earlier in the debate. Finally, Buttigieg’s response about his capability to win the election fell slightly short. He appeared flustered to me, and this was the first time I’d seen it in a debate.

C

Tom Steyer

Steyer had his best debate, and made some good points. As I have noted in past debates, his incessant staring at the camera is off-putting, especially since he’s the only one doing it. Steyer needs to remember: It’s a debate, not a podcast. However, if you were just listening to the debate and not watching it, Steyer had a lot of reasonable arguments. He’s the Richard Nixon of our time. Steyer’s a better debater on the radio than television.

C-

Joe Biden

Biden’s been unimpressive in these debates. Sometimes it’s difficult to understand his point, and sometimes he gives a meandering answer to a question about Guantanamo Bay in which he ends up talking about Israel and a two-state solution. On Thursday, Biden was asked why he couldn’t close the detention camp during the Obama administration, and after saying he needed congressional authority, he said, “Look, what we have done around the world in terms of keeping Guantanamo open or what Trump has done by no longer being an honest broker in Israel, there’s no solution for Israel other than a two-state solution.” The former vice president didn’t ever circle back to explain how his administration tried to close Guantanamo Bay.

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Anytime a debater saves their interruption for the end of a debate, I expect the next words out of their mouth to be both clear and one of their stronger points of the night. So, when Biden said he was going to interrupt on the topic of Medicare for all, I was expecting a slam dunk answer. Unfortunately, Biden simply began spouting off numbers about the cost, and while I know what he was trying to say, it appeared that he just trailed off, got interrupted, and didn’t even finish his point.

During this exchange, his floundering didn’t go unnoticed. Klobuchar tried to interrupt, and Biden should have been glad to let her do just that.