Julian Castro alone played his debate absolutely perfectly, by slamming Biden where it hurts

• Former Housing and Urban Development Secretary Julian Castro launched into a hard-hitting critique of Joe Biden's policies that veered into a veiled knock on his age.
• That's exactly how debates are supposed to work.
• Other candidates didn't like that, and tried to play it safe or scold Castro for criticizing a rival.
• They're wrong, that's a bad strategy, the polling shows that arguing with a rival has few negative consequences.
• A compelling enough critique - like Sen. Kamala Harris landed on Biden in the first debate - can solidify support as well as poach some fans from the very rival you're slamming.
• Nobody every increased their name recognition by sitting polite and quiet and off to the side.
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Earlier tonight, Julián Castro landed a serious strike on former Vice President Joe Biden, linking a mistaken description of a policy proposal to a possible lapse in memory, a thinly veiled swipe at the comparatively higher age of the former vice president. Lots of candidates - possibly in the hopes that were Biden to exit the race, they would inherit his fandom - have avoided taking such a direct swipe in a debate setting.

Read more: 'I'm fulfilling the legacy of Barack Obama and you're not': Julian Castro comes out swinging at Biden over healthcare in Democratic debate

That's not a great strategy, and here's why.

Insider has been conducting a series of recurring polls that ask who among the candidates a respondent would be satisfied with. This allows us to gauge overlapping constituencies and look at dynamics in the race that can't be captured looking at just the important topline numbers.

How the Insider 2020 Democratic primary tracker works

The last time a candidate came hard for Biden on national television, Kamala Harris was raking him over the coals for his policy on busing in the 1970s, personalizing a policy impact and landing a solid hit on the elder statesman.

Let's assume the position - one encouraged tonight by candidates like Mayor Pete Buttigieg and Sen. Amy Klobuchar - that taking a significant swipe at a frontrunning rival like Castro did is bad. If such a thing was actually bad, if a debate getting divisive and contentious had the purported negative consequences so many of the candidates seem to fear, we'd expect the following things to happen.

• After landing the attack on Biden, his supporters would clam up and reject Harris for the heresy.
• Supporters of Harris who were fond of Biden would pick a side, and she would lose some of that support.
• Harris would be seen as a pariah for going low in a political debate and would struggle to gain support.
• Candidates who played nice - like Beto O'Rourke or Pete Buttigieg - would see support as viewers reward their comportment and politesse.

Here's what actually happened after the first debate, when Harris went for Biden's jugular.

• In the five polls preceding the debate on average 59% of Democratic voters were aware of Harris.
• In the five polls that came after the debate on average 66% of Democratic voters were aware of Harris
• In the polls before the debate, 83% of voters knew of Biden. In the polls after, 84% did.
• In those polls before the debate, 39% of Biden supporters were also satisfied with Harris.
• In those polls after the debate, 48% of Biden supporters were also satisfied with Harris. That's a 9-percentage point increase. That's enormous.
• In the polls before, 67% of Harris supporters also like Biden. Afterwards, 62% did.
• Also the percentage of people who knew of Harris that were satisfied with her as nominee jumped 7 points.
• Beto, who kept his hands clean, saw his awareness jump one percentage point. For likely unrelated reasons the percentage who were satisfied with him fell 8 percentage points.

So by the mere act of getting in the fight with the person liked by the most people, Harris increased her name recognition, she presented herself as a rival to Biden and wooed a number of his supporters even if they didn't let go of their top choice, and she also locked down more of her supporters who had been flirting with Biden. Her rivals, who were behaved, were unaffected.

Debating at debates is good, who could have guessed it!

For Castro, nobody's in a better position to land a clean hit on Biden than this former Obama administration colleague. And Castro has so much to gain and very little to lose. Sure, about two thirds of his fans like Biden, but remember there seems to be very little risk on that front.

Only 17% of those satisfied with Biden are also satisfied with Castro, according to the latest data. That's really low - 11 percentage points lower than his overall satisfaction rate among Democrats - and a prime opportunity. Castro realized that debates are a great chance to mix it up, because only by mixing it up do you have other candidate's fans seriously pay attention to you.

This is why Mayor Pete Buttigieg's scolding and Sen. Amy Klobuchar's reticence to directly attack Biden - who has all those moderates she wants to voter for her - is all the more frustrating.

• Fully 75% of Klobuchar's supporters also like Biden.
• A mere 17% of Biden supporters also like Klobuchar. That is below her overall performance among Democrats by 10 points.
• A third of Biden supporters like Buttigieg, but that's a whopping 16 points below his overall performance among Democrats.
• Buttigieg's focus on Warren makes a bit more sense in context when you realise that they are far from ideological rivals, but more that 75% of Buttigieg supporters would also be satisfied with Warren as a nominee, making her a fairly immediate threat.

This may be a wasted opportunity. There's another debate for each of these middling contenders, but no guarantee they ever share a stage with the frontrunner again.

Klobuchar and Buttigieg played nice at a time when it may have been optimal to land a solid blow. Castro - and to a lesser extent Booker, but moreso after - saw an opportunity and went for it. This isn't a mixer, it's a debate, and it's not a roundtable conversation, there's an expectation that things get political. While some may see an advantage here in playing it cool and not coming off as feisty, that's not actually how debates work. Only a few of the candidates have realized that, and it'll soon be too late for the rest if they don't.

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