The most devastating rejoinder that has ever been offered during one of the debates we're now accustomed to seeing every four years. And the person that offered it lost.

It takes a certain sort of magic for a presidential debate to shift a race, it seems, some weird alchemy of ingredients such as viewership, mistakes, perceptions and medium. It's hardly ever about policy: Stuart Stevens, former aide to George W. Bush and Mitt Romney, plays down the importance of focusing on policy for candidates; The Atlantic magazine's James Fallows reminds us that policy has rarely offered the memorable moment, even recommending watching with the sound off. Instead, it's often something intangible. A sigh. A bit of dismissiveness. A glance at a watch.

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So what are the chemicals we already know will be at play?

Viewership has increased in recent cycles. According to Nielsen, the presidential debate that was viewed in the most households in history was President Obama's disastrous first debate against Mitt Romney (more on that later). About 46.2 million homes watched that debate — with the second and third debates of that cycle occupying two other places in the top 10.

As a percentage of households in America, the leader is the third debate of the 1960 election cycle, when more than three-fifths of American households turned in. But after a lull 20 years ago, viewership has rebounded.

First debates usually get more viewers than later ones. In terms of raw viewership, first debates usually overperform. In each of the years for which Nielsen has data, only in 1984, 1992 and 2008 were the first debates not above the average for all the debates. (In 1960, the third debate was also the most-watched.)

If you're trying to guess when you can anticipate the most people watching, you'd likely guess that it would be on Monday.

We saw this happen in the Republican primaries. Coming into the Republican debates in August last year, Donald Trump had seen a huge surge in support, meaning he'd appear at center stage during the contest. It was not clear what that would do to ratings, though it seemed safe to say that they'd increase over past primary debates. As we wrote at the time, that first debate blew the record away — and in fact ranks among the most-watched shows ever to air on cable.

It got more viewers than the "Walking Dead" premiere the prior year and more than the most-watched primary debate to that point plus the premiere of "Celebrity Apprentice" (which we figured might roughly approximate a GOP primary debate plus Trump).

The first debate seems to have moved the needle with some regularity in recent elections. We pulled RealClearPolitics polling averages from 2004 through 2012 and created a weighted average for 2000 from tracking polls that year collected by PollingReport. Then we overlaid the three debates in each cycle.

In 2000, 2004 and 2012, there was significant moving in the poll averages a week after the first debate. When Al Gore huffed and groaned his way through the first contest with George W. Bush in 2000, Bush picked up three points in the margin between the two of them over the next seven days. Four years later, he got demolished by John F. Kerry in the first debate — and over the next week saw his lead shrink by four points. In 2012, there was that Romney-Obama contest. Obama gave up his lead entirely, losing 4.6 points in the race against Mitt Romney.

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As the columns on that graph may remind you: More households were tuning in for that matchup than ever before. In 2004 and 2012, more than 60 million people were watching, compared with just fewer than 50 million in 2000. The shifts in the polls those two years was bigger.

Conditions, in other words, seem ripe for something dramatic — even before we add two other ingredients.

There are more undecided voters than usual. More than 10 percent of voters do not choose one of the two major-party candidates in RealClearPolitics' head-to-head average — a substantially higher percentage than in the past few election cycles.

They may never pick one. The historic unpopularity of this year's candidates may mean that many of these voters end up sitting the campaign out. Or they may be waiting for more information before making a choice — waiting, that is, for a moment like the first debate.

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The debate has an obvious wild card. Donald Trump has proved willing to say just about anything, though his appearances in past debates have been a bit more contained than some of his speeches. Trump will be looking to shake up the race by saying or doing something dramatic, and Hillary Clinton will be watching and waiting to respond once he does. There's a sense of free-for-all here that does not normally exist around presidential debates, an apparent unusual willingness to step around the typical boundaries of such contests.