Former Vice President Joe Biden had precisely the night that he needed in the 10th Democratic primary debate on Tuesday. With frontrunner Bernie Sanders on the verge of breaching his South Carolina firewall on Saturday, and with this possibly being the last debate before voter preferences solidify and the race takes on a momentum of its own, Biden needed a forceful, decisive performance. And he just might have gotten it.

Even though the ever-reliable Sanders turned in a typically workmanlike performance, the unusually lucid Biden clearly gained the most. He forcefully pushed back against the other candidates, as well as the moderators. In the midst of the coronavirus crisis and as the stock market bled out trillions in the span of days, his foreign policy experience suddenly loomed large. And for once, he came into the debate with a real strategy: zero in on billionaire Tom Steyer, who could be sucking double digits out Biden's margin in South Carolina. Win the night, win on Saturday, and worry about the rest later.

He's not wrong. Biden isn't just running out of time before the enormous delegate hauls of Super Tuesday — he's also running out of debate minutes to make his case to persuadable voters. This is true for everyone, of course, including Massachusetts Senator Elizabeth Warren, Minnesota Senator Amy Klobuchar, and former South Bend Mayor Pete Buttigieg, but Biden is the only one who has basically admitted he's toast if he doesn't win South Carolina.

One of the ironies of the modern nominating process is that the frequency of debates increases just as their efficacy in changing minds craters. Many undecided voters have already made up their minds, and once people make a firm choice in these primaries, they tend to dig in and tune out appeals from other candidates. That means that even though the South Carolina debate took place just six days after the one in Nevada, fewer and fewer viewers are even open to listening.

While there is a robust research program looking at the effects of general election debates on presidential election outcomes (the quick-and-dirty summary is that they don't seem to matter much at all), relatively less work has been done by political scientists and communications scholars on the primary debates currently consuming so much oxygen in the Democratic primary. But what little work there is suggests that the earlier debates have greater effects.

Early in the process — the summer and fall before the election year — Democrats and Republicans have mostly held once-a-month affairs as they wait for the field to winnow itself, before a flurry of frequent debates as the early state contests commence. In the 2016 Republican primary and the 2020 Democratic primary, the number of presidential hopefuls has been so large that the parties have had to scramble to create two separate debates. Despite the way that these gabfests might have felt unpleasant to viewers overwhelmed by crosstalk and brief response times, they created many opportunities to change the dynamics of the race.

In 2016, a group of researchers at the University of Missouri led by Benjamin Warner had students watch four debates — one for each party in the fall and the spring. The numbers they produced were eye-popping. After watching the October 13, 2015 Democratic debate between Hillary Clinton, Martin O'Malley, Lincoln Chafee, Jim Webb and Bernie Sanders, for example, students more than doubled their support for O'Malley. Out of 151 viewers, Clinton lost 15 and Sanders 3. Overall fully 29 percent changed their candidate preference. By the time 94 self-identified Democrats watched the March 9, 2016 debate in Miami, their positions had hardened — just 10 percent changed their mind after viewing, with neither Clinton nor Sanders (the surviving candidates) meaningfully increasing their backing.

In the more crowded GOP field, fully 54 percent of students who watched the October 28, 2015 Republican debate in Colorado changed their minds about who to support after viewing, with Florida Sen. Marco Rubio nearly tripling his polling. Physician Ben Carson and former Florida Gov. Jeb Bush saw their support cut in half. The eventual nominee, Donald Trump, saw a modest drop in support after both debates, not surprising with this sample of very young Republicans who remain much less enthusiastic about the president than their older peers. The effects of the GOP's spring debate were comparatively modest but definitely more significant than on the Democratic side, with more than a third of viewers switching candidates.

This was just one study, and some social scientists can be skeptical of research that depends so heavily on your own college students. The limited data that we have confirms that candidate-switching plummets as the campaign unfolds, which makes sense on an intuitive level. After all, the longer you've been thinking about the race, and the more you've seen of the candidates, the more likely you are to collect the information you think you need to make up your mind. The candidates you found mildly off-putting five months ago now make your skin crawl, and you yearn for the day that you are liberated from ever having to listen to them speak again.

These conclusions hold up pretty well when measured against the current Democratic nominating cycle. After the first debate, there were wild swings in support for some candidates. California Sen. Kamala Harris, for example, nearly tripled her polling average after she confronted former Vice President Joe Biden about his past opposition to federally-mandated busing in the June debate. Warren, Buttigieg, and Klobuchar also all saw significant spikes in their support after particularly effective debate performances early on in the process. So while they might occasionally be insufferable, these shouting matches do serve a real public purpose in that they offer people an easy and accessible way to make choices.

At a certain point though, the results of voting in the early states becomes much more determinative than any further debates. You can't erase a delegate deficit with a clever retort, and even a robust performance is unlikely to challenge narratives that have calcified over the course of an already long and grueling campaign. Early success or failure can profoundly affect whether viewers see candidates as winners or losers, electable or unelectable, viable or unviable, in the later debates. Elizabeth Warren's inability to translate her exceptional debate performance last week into results in Nevada is a case in point.

If Sanders wins South Carolina, it could effectively end it all. And that's why Biden probably felt some urgency. This could really be the last time he could've taken his case directly to an audience of millions with a chance to change a sufficient number of minds. The next scheduled debate isn't until March 15.

There are a few factors, though, suggesting that Democrats might have one or two more influential debates left. First, voters got their first look at a significant new candidate last week, former New York City Mayor Michael Bloomberg, who has spent mind-gobbling stacks of scratch advertising across Super Tuesday states and beyond to prop up his candidacy. That means that his bus-wreck of a performance in the Nevada debate likely had real costs for him. Still, his introduction to the debate stage so late in the process means that some more mid-primary volatility should be expected than in 2016 or 2008.

The Democrats have also not yet gone through what the GOP did in 2016 — see a true party outsider seize control of the nomination. That is not at all to morally equate Trump with Sanders, but rather to suggest that the process of coalescing around an obvious frontrunner and seeing support snowball over the course of successive primary wins might not be where we're headed. The unknowable dynamics of the moderate effort to stop Sanders from capturing the nomination should invite some humility when seeking to apply lessons from the past to the immediate future.

That said, this could very well have been the last meaningful primary debate of the 2020 cycle. If it was, Biden did everything he could to put himself back in serious contention for the nomination.

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