Dan Guild is a lawyer and project manager who lives in New Hampshire. In addition to writing for Bleeding Heartland, he has written for CNN and Sabato’s Crystal Ball, most recently here. He also contributed to the Washington Post’s 2020 primary simulations. Follow him on Twitter @dcg1114.

This post updates a piece I wrote in 2020.

At this moment the race between Joe Biden and Donald Trump is close; you could argue it is the closest in U.S. history. To say it is unique is to state the obvious. This is the first presidential campaign in the modern era where both candidates have held the office of president. It is unique in another way too: many Americans did not want this race.

As the data below shows, incumbents typically do poorly in the first debate. If that trend holds this year, it bodes ill for Biden—but this time may be very different.

The reason challengers tend to win the first debate is the event allows them to consolidate their partisan support. We saw this happen in 2012, when Mitt Romney gained significantly against President Barack Obama, simply by increasing his margin among Republicans and Republican leaning independents.

This time, though, it is Biden who has struggled to consolidate his party’s base.

This graphic comes from Gallup, though I could cite other data as well. These findings highlight the opportunity—and the danger—for the president. A good performance tonight may allow Biden to close the gap below. A bad one and this gap may become permanent, creating enormous turnout problems among key elements of the Democratic base.

Since the advent of television, politics and history have occasionally turned on a few moments. Seldom do they last longer than 60 seconds (like wit, television values brevity above all else).

Senator Joe McCarthy, and the moment he led, were stopped when he was asked, “Have you no decency, sir?” During the Watergate hearings, Senator Howard Baker summed up the entire scandal when he asked, “What did the president know, and when did he know it?”

Some of the game-changing events have happened in debates. Ronald Reagan revived his flagging campaign in 1980 during a debate about a debate. Later that year, he uttered two phrases that seemed to sum up the entire election. “There you go again,” he said in response to President Jimmy Carter’s attack on his position on Social Security. (During Reagan’s presidency, Carter would be proven right.)

But even more memorable is Reagan’s close, from the final debate in late October 1980:

The country would answer the question with a resounding no.

This chart summarizes the impact of presidential debates since 1976.

A couple of things to note on the day of the first Trump/Biden debate:

Debate bounces often don’t last. The debates had a pretty big effect on the last two presidential races. Hillary Clinton led by 2.6 percent going into the first 2016 debate. Her lead would climb to more 8 percent after the second debate. But her pre-debate average was very close to her final popular vote margin (2.1 percent).

The same was true in 2012: Obama led by 3.8 percent going into the first debate. He even trailed after the first debate. But in the end, his final advantage in popular vote share was nearly identical to his pre-debate average.

Incumbents beware. Obama, George W. Bush, and Reagan in 1984 all saw significant declines after their first debate. In Reagan’s case, the cause was a poor performance and concerns that he was too old. (That concern was borne out in some ways during his second term.) John Kerry and Romney consolidated their bases in the first debates of 2004 and 2012, and both races closed significantly.

Practice your one-liners. “There you go again.” “You sir, are no Jack Kennedy.” The best punch is one short enough for a TV news clip.

Sometimes it isn’t the answer, it is the question. 1988 nominee Michael Dukakis never recovered from a question about the death penalty. CNN’s Bernard Shaw asked what the Democrat would do if his wife were raped and murdered. The unusual question seemed to call for a visceral response—which Dukakis did not provide.

Debates matter. President Gerald Ford closed the gap after his first debate with Jimmy Carter in 1976. George W. Bush changed the trajectory of the 2000 race in his debates against Al Gore. It often seems as if the debates allow a candidate to consolidate their party’s vote. That certainly happened in 1976 and 2012.

These are the polling averages before and after each debate. In some years it is not possible to differentiate between polling after one debate and before the next one.