It’s a natural default assumption that when big-ticket events occur over the course of a presidential election, they are necessarily important. Everyone gets together for a debate that’s watched by millions of people; how could that not shape the outcome of the race?

But a funny thing happened in 2016. From the date of the first sanctioned debate by either party until both nominees clinched the necessary delegates, there was a period of only three days in which Donald Trump or Hillary Clinton didn’t lead in the polls. Trump and Clinton both faced threats at various times, but both essentially went wire to wire through the entire campaign.

As debate season looms in the 2020 contest, it’s worth considering how those encounters changed the field in 2016, particularly because the dynamics are similar. Then, as now, one party had a crowded field with a diverse array of candidates, one of whom had a decent-size lead. Then, as now, the other party had a prohibitive front-runner.

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Here’s what happened when they started to debate.

The Republicans

To accommodate the crowded Republican field in 2016, the party created a two-tiered debate structure. The 10 candidates faring best in polls would participate in the main debate. Other candidates who’d met a minimum polling standard participated in a secondary debate that aired before the main event.

As Aug. 5 approached, the jockeying was mostly to see which candidates would make the main stage and which would be forced to sit at the kid’s table. This involved a lot of strategy: Then-Gov. John Kasich (Ohio) entered the race late, got a nice bump from his announcement and slid into the debate.

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But it was one of the candidates from the lower-tier debate who saw the most movement afterward. Looking at the RealClearPolitics poll average by day, we see that the two candidates who gained the most in the polls in the weeks after the debate was held were Ben Carson and business executive Carly Fiorina.

Carson gained more than four points in the three weeks after the debate, moving from fifth place into second. Fiorina was a standout in the junior debate, gaining slightly more than Carson and passing several of the candidates who’d participated in the main event.

The next debate came a little over a month later. This time, there was a lot more movement in the weeks after the debate. It takes a bit for changes to register in the polls, given that polling isn’t instantaneous, but you can see the shifts on the graph below within the first 10 days or so.

Fiorina, now in the main debate, gained more than eight points during that period. Carson slipped a bit, but not as much as Trump, who lost more than six points. It was obvious during the event that Fiorina outplayed Trump, a result reflected in the polling.

Perhaps the most interesting thing to occur in the wake of the debate, though, was the withdrawal of then-Gov. Scott Walker (Wis.). Walker had at one point led the field. But without making headway in the debate and with a campaign that had quickly hit rocky financial waters, he called it quits.

A month and a half later, the third debate. Again, Trump came out slightly lower than he’d gone in, and, again, Carson gained. You can see on the graph below the result: For a few days in early November, Carson took the lead in national Republican primary polling.

Notice, too, that Fiorina’s success didn’t last. She never even reached half of Trump’s support in the wake of the debates, and, by January, she’d been dropped back down to the secondary debate. (Those aren’t shown above because no one ever replicated her success from that first event.)

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The lead by Carson wouldn’t last. Not because of a debate, though. It was because of France.

Specifically, the terrorist attacks that occurred in Paris in mid-November. Trump and Carson saw their poll numbers diverge after the fourth debate, but more so after terrorists killed more than 130 people in the French capital. Carson’s strength wasn’t foreign policy, and Trump seized on the attacks as reinforcement of his key political messaging on the purported threats of terrorism and migration.

That, as it turns out, was about the end of that. A debate in mid-December doesn’t seem to have changed the momentum of the race much at all, with Sen. Ted Cruz (R-Tex.) picking up support that Carson was shedding.

A debate a month later, a few weeks before the caucuses began in Iowa, had almost no effect on the field.

By late January, Trump’s irritation with the debates was beginning to show. When Fox News was slated to host a debate in late January — the network’s first since the opening debate in which Trump was a target of direct questioning — Trump decided to boycott. Instead, he hosted a fundraising/campaign event focused on the military in Des Moines. (That event helped doom the Trump Foundation.)

It’s hard to tell how much the pared-down debate shifted the flow of the campaign because Iowa began voting days later. It was those results that probably had the most significant effect on the field, with Sen. Marco Rubio (R-Fla.) surging after his surprisingly strong finish in the caucuses.

The next debate had one of the most memorable moments of the cycle. Rubio, hoping to capitalize on the attention he gained after Iowa, went after Barack Obama.

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“Let’s dispel once and for all with this fiction that Barack Obama doesn’t know what he’s doing,” Rubio said. “He knows exactly what he’s doing.”

Then he said it again. And again — even after then-Gov. Chris Christie (N.J.) called him out for sounding robotic.

The debate doesn’t seem to have changed much. Rubio fared poorly in the New Hampshire primary, but his campaign also wasn’t doing much there.

Christie dropped out shortly after. Rubio faded, though he’d later briefly surge again. Kasich did fairly well in New Hampshire and gained in the polls.

By this point, though, it was the voting that was clearly winnowing the field. By mid-March, leading in votes and delegates, Trump essentially refused to participate in any more debates. He went on to win the nomination.

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The Democrats

On the Democratic side, the pattern was even less interesting.

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During the candidates’ first sanctioned debate, Sen. Bernie Sanders (I-Vt.) offered the most memorable line of the Democratic contest — a line that helped his opponent.

“I think the secretary of state is right,” Sanders said, referring to Clinton. “The American people are sick and tired about hearing about your damn emails.”

The American people would soon hear much, much more about Clinton’s private email server, and details of the FBI’s investigation into the server probably helped Trump win the 2016 general election.

But after Sanders offered that line, it was Clinton who gained in the polls, picking up eight points in the three weeks after the debate, while Sanders remained fairly flat.

Remember, though, that Sanders had already gained a lot of ground against Clinton. In spring 2015, her lead topped 50 points. By October, it had narrowed to less than 20.

A second debate in November didn’t change things much.

Nor did the third.

By mid-January 2016, Sanders had pulled close to Clinton in national polling. The debate Jan. 17 didn’t change that much.

As with the Republican field, Sanders came closest to surpassing Clinton once voting began.

Sanders saw sporadic surges in support over the course of the primary election cycle, but they don’t seem to have been connected to the debates. In short order, there were so many debates and candidate forums, overlaid with state contests, that identifying cause and effect becomes tricky. Sanders’s spikes often preceded these events.

By the ninth debate in mid-April, Sanders had essentially tied Clinton in the national polling. Again, polling after that event doesn’t seem to suggest much movement.

But by then, the issue was largely moot. By mid-March, Hillary Clinton had built a large-enough delegate lead that, given the distribution of delegates in the Democratic process, Sanders essentially couldn’t catch her.

Hillary led the entire time.

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