Donald Trump had room to be messy in the debate. He’s not a career politician. He is the change candidate. The disrupter. His roughness and “stumbles” are proof of concept. It’s a good thing he doesn’t know how to behave like the disappointing career politicians. But there is a limit to this line of argument. Change is a haircut; too much change is a decapitation.

In the first debate Donald Trump had his best opportunity to show that he could be himself while adhering to a minimum set of requirements. He missed the opportunity. The debate required preparation beforehand and deliberation in the moment. His lack of preparation showed and in the cut and thrust with Hillary Clinton, Trump could not was not still his impulses. If he can’t do it in a campaign and in a debate, how would he be able to do it in the White House? Debates have not often changed the trajectory of presidential races, but if voters do turn away from Trump, it may be because his performance so directly illuminated his key vulnerability. If they don’t, it will show just how much voters want change for change’s sake.

Polls have shown that a consistent majority of those surveyed think Donald Trump is not prepared to be president. Recently, Trump has been making a more concerted effort to answer this worry, particularly with college-educated Republicans he needs to be a part of his electoral coalition. He’s been using a teleprompter and avoiding the most obvious politically damaging fights. His campaign advisers believe this has paid off with voters who dislike Hillary Clinton and don’t need to see a wholesale change of heart. They are anxious to conclude, based on just a little more campaign hygiene, that Trump has the judgment to be president. They want to believe.

Trump started the debate in pretty good form. He relentlessly framed Clinton as a part of the political class that had not been able to solve the country’s problems. Over the 90 minutes, his answers started to fray, though. His answer on the use of nuclear weapons was particularly meandering. His asides about his son’s computer skills and 400-pound hackers added to the feel that on some answers he was rummaging through the cupboard for anything he could find. When he tried to explain why he would not release his tax returns or why Sean Hannity held the key to his opposition to the Iraq war, his answers were hard to follow and strained.

To clear the presidential threshold Trump didn’t need to have mastery, but his claim throughout his campaign has been that when the time comes, he will get up to speed and know more than any expert in the field. The time came. He did not meet his boast. It was the opposite of Arnold Palmer’s advice. “Don’t talk about how good you are. Show them.”

When you brag about how much you will know and be able to demonstrate and then don’t do the work, that’s not the result of inexperience: that suggests a larger flaw.

When put in a spot over difficult positions, Donald Trump repeatedly said things that were not so. He denied that he had called climate change a hoax cooked up by the Chinese. He did. He denied that he had said pregnancy was an inconvenience for business. He had. He said that he opposed the Iraq war. He did not in public. (He and supporter Sean Hanity say he was against it in private). He denied that he said Hillary Clinton did not have the presidential “look.” He did. These are not matters of opinion. They are verifiable because he is on the record as saying something different in the past than he was saying in the debate.

Perhaps the most extensive example was his defense of his five-year advocacy of the idea that Barack Obama was not born in the United States. He said Hillary Clinton started the rumor, which is not true, that he was pursuing the hunt to solve the mystery, which is not true, and that the matter was resolved for him when the president submitted his birth certificate, which is also not true. He then added that the African American community was happy that he had spent five years on this mission.

Hillary Clinton fudged the truth a few times in the debate as well, on her history of advocacy for the Trans Pacific Partnership, but, as the Washington Post put it, “ her misstatements paled in comparison to the list of Trump’s exaggerations and falsehoods.”

Usually the question of temperament is an abstract one in debates. Viewers this year got a chance to watch the temperament of both candidates in real time. Toward the end of the debate Trump increasingly interrupted Clinton -- which might not be a problem if another candidate did it -- but when impulse control is your burden, it’s a best practice to keep the interjections low. Trump knew rudeness was a liability because early in the debate he stopped one of his answers to make sure that the audience recognized that he had complimented Clinton.

Clinton fans are passing around the compilation of Trump interruptions, her defense of the women Trump has insulted and the back-and-forth over her stamina. Debates are a chance for campaigns to stir the faithful. Though Clinton’a advocacy for her own policies had little lift or inspiration, she left the stage with several clips that could be passed around on social media to rally her troops. Trump’s troops are already in a permanent state of rally. They’re not likely to leave the Trump train after his debate performance. But if he wants to get more people aboard, the conductor is going to have to show that he can keep things on track.