Michael A. Cohen (NOT the ex-Trump lawyer) writes that this debate might change the views of independent, uncommitted voters. Trump’s behavior and Harris’s cool were a stark contrast. Republicans are complaining that the moderators fact-checked Trump but not Harris, and were biased. But a few of Trump’s many lies were so egregious that the moderators were compelled to correct him, such as his debunked claim that Haitian immigrants in Springfield, Ohio, were stealing and eating pets, and his claim that Democrats support “post-birth” abortion. The moderators pointed out that the pet story was a hoax and that no state allows murdering a baby after birth.

Cohen writes:

Presidential debates usually don’t matter. A trove of political science literature suggests that most debate watchers have already decided whom they are supporting. While a winning candidate might get a temporary boost from a strong performance, the polling bump often fades. 

However, last night’s showdown between Kamala Harris and Donald Trump could be the exception to the rule. Why? Because never before in American presidential politics has there been a debate as one-sided as what we saw Tuesday night in Philadelphia. 

If this had been a heavyweight fight, a towel would have flown across the ring and the referee would have stopped the bout. This was such a rout that even conservative pundits bemoaned Trump’s disastrous performance. Over and over, Harris threw fresh chum into the water. In practically every one of her answers, she included at least one line that she knew would firmly lodge itself under Trump’s infamously thin skin. 

She needled Trump about his boring political rallies and pointed out that his alma mater, the Wharton School of Business, had thrown cold water on his economic plans. She listed his litany of criminal indictments and prosecutions. She repeatedly called him a disgrace and an easy mark for foreign leaders.

And each and every time, without fail, Trump took the bait. The result was a series of angry, disjointed and incoherent rants at ever-increasing decibel levels. He claimed without evidence that “many of those [Wharton] professors … think my plan is a brilliant plan.” He defended his political grievance fests by claiming they are the “most incredible rallies in the history of politics.” And in the debate’s most bizarre moment, he falsely claimed that immigrants in Ohio are stealing and killing pet animals. The contrast between sullen, angry Trump and polished, even-keeled Harris couldn’t have been starker. While much of the analysis from last night will focus on Trump’s lunacy, Harris’ performance may have been more decisive.

By and large, voters know what they think about Trump. Nine years in the political spotlight will have that effect. But Harris has been a 2024 presidential candidate for just seven weeks. If recent polling is to be believed, going into last night many voters saidthey want to know more about her. In a New York Times poll released Sunday, 28 percent of voters “said they felt they needed to know more about Ms. Harris, while only 9 percent said they needed to know more about Mr. Trump.” The number is close to half among the small segment of undecided voters. Along with last month’s Democratic convention, Tuesday’s debate was one of Harris’ best opportunities to introduce herself to the public. Did last night seal the deal? CNN’s instant poll taken immediately after the debate showed Harris trouncing Trump 63-37. That’s almost a mirror image of its poll after the Biden-Trump debate earlier this year. It’s similar to the margins for Joe Biden, Hillary Clinton and Mitt Romney in the first debates of the last three presidential elections — each of which led to a bump in the polls.

But a strong debate performance is no guarantee of victory. In 2004, John Kerry trounced George W. Bush in all three presidential debates. The same was true for Clinton against Trump in 2016. In 2012, Romney wiped the floor in his first debate with a listless Barack Obama. None of those three ended up in the Oval Office. 

Still, the differences between Harris and Trump were so significant — and considering the potential boost to a candidate not as well known as her opponent — it’s hard to imagine last night’s debate will not have at least some effect on voter opinion. At the very least, she might have given the sliver of the electorate still unsure about Harris enough information to win their vote in November.In the near term, the debate should generate days of coverage about the former president’s mental state. Perhaps it will also move the news media away from continuing to claim that Harris has not explained herself and her plans to the American people.

But ultimately, the question for Democrats is: Did Harris swing enough voters in her direction to ensure she wins the White House? Even if her poll numbers improve in the next week, will those gains remain in place until Election Day?

Time will tell. But if Trump remains a high-floor, low-ceiling candidate, with a strong base of support and a limited ability to bring in new voters, even a small move of undecided voters to Harris could be decisive. And it’s hard to imagine any presidential candidate having a better night than Harris did on Tuesday. Democrats can’t ask for much more than that from their new standard bearer.

Michael A. Cohen

Michael A. Cohen is a columnist for MSNBC and a Senior Fellow and co-director of the Afghanistan Assumptions Project at the Center for Strategic Studies at the Fletcher School, Tufts University. He writes the political newsletter Truth and Consequences. He has been a columnist at The Boston Globe, The Guardian and Foreign Policy, and he is the author of three books, the most recent being “Clear and Present Safety: The World Has Never Been Better and Why That Matters to Americans.”