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Don't dismiss tonight's GOP debate — history says it's likely to change many minds

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Update: Coverage of second GOP debate on CNN.

General elections — the big showdown between the Democratic nominee and the Republican nominee — attract more interest than the primary campaigns that pick them. That goes for debates, too. More than 67 million people watched Barack Obama debate Mitt Romney in 2012, while that year's GOP primary debates attracted between 3 million and 7 million viewers each.

But even though fewer people are watching, the evidence is pretty clear that primary debates matter more. The policy differences between the candidates are much smaller, and many highly engaged people are genuinely undecided. So primary debate performances can swing outcomes in a way that's not true for the general election.

General election debates don't seem to matter much

Political lore is full of dramatic debate moments and campaign turning points. But while it's certainly true that a good debate performance can offer a candidate several days of favorable media coverage, there doesn't seem to be much enduring impact, and there's no evidence of any decisive debate moments.

"Scholars who have looked most carefully at the data," writes George Washington University's John Sides, "have found that, when it comes to shifting enough votes to decide the outcome of the election, presidential debates have rarely, if ever, mattered."

In their book The Timeline of Presidential Elections: How Campaigns Do (and Do Not) Matter, Robert Erikson and Christopher Wlezien offer the following chart showing a near-perfect correlation between pre-debate and post-debate polling:

This is a bit of a bummer for debate fans. But it also makes a lot of sense. General election debates feature a contest between the leaders of two political parties that offer systematically different policy agendas. A person who favors low taxes and thinks abortion should be illegal isn't going to change his mind just because Barack Obama gets off some good zingers. A person who's passionate about environmental protection and LGBTQ rights might be dismayed by a weak debate performance by a Democrat, but he's not going to vote Republican.

Further entrenching this dynamic is the fact that the people who watch the debates are disproportionately people who have already made up their minds. That's because debates are mostly boring unless you're interested in politics, and people who are interested in politics don't need to watch a debate to make up their minds.

But primary debates are different

The primaries are very different. A typical primary field offers multiple candidates who conform to the same key ideological tenets. Contenders with heterodox views in their records tend to trim their sails in an effort to better conform to the party's agenda. A Republican looking at the 2012 race can't just decide to support the guy who wants low taxes and more restrictions on abortion, because they all want that. The subtler differences highlighted by a debate can really sway support, and even the kind of highly engaged voters who are likely to watch a debate can have their minds changed.

A study by Mitchell S. McKinney and Benjamin R. Warner found that a whopping 60 percent of primary debate viewers expressed a change in voting intention as a result, including 22.6 percent who come in undecided and 35 percent who switch between candidates.

Primary debates are about more than winning

Debates also matter because there's a lot more going on than a simple contest for voters' affections.

Bernie Sanders is extremely unlikely to win the Democratic nomination, but he's already succeeded to some extent in shaping the party's internal conversation about its medium-term future — a strong debate performance is a chance to grow that influence, while a weak one will reduce it.

On the Republican side, Ted Cruz will be debating not so much to win the nomination as to try to fight for renewed relevance as an attention-getter in a Donald Trump world. He has a few big donors, a small but active nationwide fan base, and many admirers in the conservative media. Strong debates build that fan base and the party those fans support, while weak performances will weaken them. By the same token, Marco Rubio is simultaneously trying to displace Jeb Bush as "the electable guy from Florida" in the GOP field and seem like the guy who should be vice president if Scott Walker wins the nomination.

Last but by no means least, Walker and Bush are going to be subject to the glare of sustained national attention for the first time. Party actors who don't have incredibly strong feelings about immigration will mostly be looking to see who handles the pressure better. Can Jeb deliver on the idea that he's the more polished, more professional, more ready candidate — or will he look a little rusty and out of touch after having last run a campaign in 2002?

Expecting single moments to be decisive in a process as complicated as a presidential nomination is unrealistic, but the stakes are genuinely high. Not many people watch primary debates, but if you care about who wins the Republican nomination you really should.