We seem to be approaching something of a punditry consensus that Hillary Clinton bested Donald Trump at their first debate on Monday night. But just what kind of effect, if any, should we expect this to have on the election? We know that presidential debates, even if they're considered very one-sided, rarely have a large impact on the election itself. But this has also been such an odd year. What might we expect?
Below, I outline the cases for no effect, a modest effect, and a large effect.
In those races where debates seem to have moved the needle a bit, it's usually because they provided information about the candidates that we didn't have before. Ronald Reagan's steady performance in his one debate with President Jimmy Carter in 1980, for example, caused many voters to rethink their views of the former as an unprepared hothead, helping him substantially in the polls.
In 2016, however, the major party candidates are very well-known, having been in the public eye for decades. What's more, they didn't really behave in surprising ways. Clinton was cool, prepared, and knowledgeable; Trump was aggressive, brash, and confident. In other words, they acted exactly like they did in the primary debates in 2015 and early 2016.
No one who watched those earlier events saw much that was new on Monday. Yes, Trump took some criticism for coming off as sexist and disorganized, but his supporters were largely already comfortable with such behavior by him. So unless there's something new here, we really shouldn't expect to see the polls shift.
Modest effect (1 to 3 points)
As Nate Silver noted, candidates who are perceived as doing better in the immediate post-debate polls tend to get a modest bounce in the general election matchups. Clinton substantially outperformed Trump in those early polls, which will probably help her campaign.
Moreover, news coverage of the debate has stayed pretty similar in tone over the past few days. Unlike in 2000, when an early perceived win by Al Gore became a perceived win by George Bush a few days later, coverage has continued to focus on Clinton's strengths and Trump's weaknesses. Thus, given how polls have behaved in the past, we can expect a modest bounce for Clinton.
Large effect (4 points or more)
Our understanding of debate effects, as suggested above, relies on examples from previous presidential campaigns. But as we are reminded over and over again, Trump is not running a typical presidential campaign.
What happens in a typical campaign? If a candidate has a bad debate performance, he or she tends to try to immediately change the subject to an area of strength.
President Obama's performance in his first debate with Mitt Romney in 2012, for example, was widely panned, and audiences credited Romney with a substantial win. The very next day, Obama's chief campaign strategist, David Axelrod, gave a press conference conceding Romney's strong performance in the debate while attempting to steer coverage toward discussion of Romney's honesty, an area of perceived weakness for the Republican nominee. The Obama campaign released an ad on that topic the same day.
The damage had been done — Obama lost a few points to Romney that week — although it proved temporary.
This is where Trump differs from a normal candidate. He is still litigating his areas of weakness from Monday's debate. He's not changing the subject. If anything, he's exacerbating his loss. He got stung late in the debate when Clinton brought up the example of Alicia Machado, a beauty pageant contestant whom Trump had insulted. Instead of trying to change the topic, he continued to berate Machado for her weight the next day.
He also continued to say Clinton lacked the "stamina" to be president, even after she effectively parried him on the issue and after their 90-minute debate performances substantially undermined his claim. And he complained that the moderator was biased against him and that he had a defective microphone.
This is not how one changes the topic. Trump is doing his opponent's work for her. But this is entirely consistent with his behavior right after the Democratic National Convention in July, when, instead of simply staying silent or trying to change the conversation, he chose to pick a very public fight with his most sympathetic critics, Khizr and Ghazala Khan. He only ended up magnifying the damage their criticisms did to him, helping give Clinton an unusually large and long-lasting convention bounce.
Given this recent behavior, it's entirely reasonable to expect a larger and more durable bounce for Clinton than debate winners usually receive.
Polls from after Monday's debate are starting to come in, so we'll have a sense over the next few days of just what kind of bounce, if any, Clinton will receive. And there's pretty much no reason to expect any bounce to last through Election Day. But it's good to keep in mind not only how this election is similar to those that have come before but also how it differs.