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Editor’s Note: Julian Zelizer, a CNN political analyst, is a professor of history and public affairs at Princeton University and author of the forthcoming book, “Burning Down the House: Newt Gingrich, the Fall of a Speaker, and the Rise of the New Republican Party.” Follow him on Twitter: @julianzelizer. The views expressed in this commentary are his own. View more opinion at CNN.

(CNN) —  

As the Democratic caucuses and primaries heat up, the candidates are starting to go after each other with greater ferocity. Sensing a potential victory in Iowa, Mayor Pete Buttigieg launched an ad that goes after proposals from Sens. Elizabeth Warren and Bernie Sanders to make tuition free at public colleges and universities.

“I believe we should move to make college affordable for everybody,” Buttigieg says in the ad. “There are some voices saying, ‘Well that doesn’t count unless you go even further, unless it’s free even for the kids of millionaires. But I only want to make promises that we can keep.”

The mayor’s critics have pounced on this line of attack. After all, some of the nation’s most successful federal programs — such as Social Security, Medicare and K-12 public education — have been available to all Americans regardless of their income. Surely, he doesn’t think that these programs are also misconceived?

New York Rep. Alexandria Ocasio Cortez weighed in with a warning on Twitter: “This is a GOP talking point used to dismantle public systems & it’s sad to see a Dem candidate adopt it. Just like rich kids can attend public school, they should be able to attend tuition free public college.”

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To be sure, it is more than legitimate for Democrats to attack proposals from their competitors as too bold or poorly conceived. It is healthy for Democrats to encounter tough criticism from every quarter to see who is best prepared to take on President Donald Trump in 2020.

But Democrats need to be cautious. There is a long history of Republican candidates using smears that unfairly characterize candidates or their policies against the Democratic party nominee. Very often the candidates who think of themselves as the most moderate in the bunch find themselves the target of rhetoric that they or one of their colleagues originally produced.

Democratic candidates who seek to paint themselves as the most moderate in the pack should not forget just how far the GOP is willing go to paint any of the nominees as wild-eyed socialist radicals. Democrats should not underestimate the importance of working as a party to frame the debate around key issues so that they don’t end up giving bipartisan legitimacy to spurious conservative talking points.

During the height of the Cold War in the 1940s and 1950s, Democrats often went after each other by accusing others of being soft on communism. Prominent officials in the party castigated the left wing by charging that they were too radical. In doing so, they played into the strategy of equating domestic progressivism and international diplomacy with socialism. Former Vice President Henry Wallace was one of the prime victims of this kind of campaign.

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