(Bloomberg Opinion) -- The last Democratic presidential debate before the Iowa caucuses demonstrated one thing above all: The candidates’ strategists have learned the lesson of every previous campaign in which two leading candidates in a multi-candidate race engaged in serious negative campaigning, only to find both of them destroyed.

That was the story, for example, of Howard Dean and Richard Gephardt in 2004, when those two Democrats attacked each other bitterly only to see Massachusetts Senator John Kerry surge from behind to defeat them. Or, perhaps, it’s the lesson to draw from the Republican contest in 2016, when Senator Marco Rubio of Florida self-immolated when he launched strong attacks against Donald Trump before Super Tuesday. Or from the fact that the two candidates who challenged former Vice President Joe Biden most aggressively in several 2019 debates, California Representative Eric Swalwell and ex-Mayor Julian Castro of San Antonio, Texas, didn't survive.

On Tuesday night in Des Moines, Iowa, no one took on Biden, the front-runner. No one really took on anybody. Sure, there were politely framed disagreements on policy questions. Senator Elizabeth Warren of Massachusetts supported the revised version of the free-trade agreement between the U.S., Canada and Mexico while Vermont Senator Bernie Sanders opposed it. Senator Amy Klobuchar of Minnesota explained why she opposed eliminating private health insurance under a single-payer, government health-care plan while Sanders and Warren made their cases for it. Biden and Klobuchar argued for a continued U.S. military presence in the Middle East, while Sanders and Warren argued for less intervention. 

But it was never personal, or even especially heated. That was probably the correct political strategy even if it didn’t make for the most thrilling debate.

As usual, I’ll avoid proclaiming who “won” the debate. As far as that goes, what matters is what the media collectively decides — which candidates are portrayed as having done well, which clips get shown on the broadcast network morning shows and are highlighted on media Twitter feeds, which politicians get more time on the cable news networks the next day. 

I do think that each of the five plausible nominees (excluding the sixth Democrat on the Iowa stage, California businessman Tom Steyer, who made little impression) probably did what their campaigns hoped they would do. Sanders showed his enthusiasm for his self-described democratic socialist ideas. Klobuchar drew some contrasts with the others, and continues to be the only debater who regularly uses humor. She also managed (sometimes, at least) to talk like a regular person while also showing skill at deploying pre-programmed zingers, a tricky balance to pull off. Pete Buttigieg, the ex-mayor of South Bend, Indiana, seemed to me to be a bit over-programmed this time, but I suspect he succeeded in showing off the thoughtful moderation that his fans like in him. Biden avoided controversy. Warren? To me, she always sounds like the best current Democratic politician when it comes to singing the liberal songbook, and she was certainly on her game tonight. She was particularly good (and better than Klobuchar) in her defense of the proposition that a woman can win the presidency — an argument she was undoubtedly eager to make after provoking a dust-up with Sanders over the matter of female political potency.

What distinguished Tuesday’s debate from others earlier in the campaign was that in the aftermath of the recent Iran crisis, the moderators started with foreign policy and national security and stayed with it for a while. My impression was that Biden and Klobuchar are the only leading contenders who are deeply interested in the topic. Sanders recites slogans and can legitimately brag about being correct about Iraq (and Vietnam, for that matter). Warren can speak coherently on virtually any public policy topic, while Buttigieg can lean heavily on his military record as a naval intelligence officer who served in Afghanistan. But none of those three manages to convey, at least as I hear them, anything like the passion that Sanders and Warren display for domestic policy. What I’d like a better sense of is whether they have command of foreign-affairs detail, but that’s probably too much to ask these stylized debates to deliver.

Polling in advance of the Feb. 3 Iowa caucuses is predicting a close contest between Biden, Sanders, Buttigieg and Warren, but the state has a history of substantial late swings. Watch carefully in the aftermath of the debate: If the media do settle on a single “winner” over the next couple of days, that could push one of those four to the top — or help Klobuchar gain the support she would need to join the top tier.

To contact the author of this story: Jonathan Bernstein at jbernstein62@bloomberg.net

To contact the editor responsible for this story: Jonathan Landman at jlandman4@bloomberg.net

This column does not necessarily reflect the opinion of Bloomberg LP and its owners.

Jonathan Bernstein is a Bloomberg Opinion columnist covering politics and policy. He taught political science at the University of Texas at San Antonio and DePauw University and wrote A Plain Blog About Politics.

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