One factor in Sanders’s success is how little scrutiny he has faced from rivals on the campaign trail and the debate stage. Media accounts that catalogue Sanders’s atypical history and decades-old comments are easy to find for anyone who cares to look. But no one knows how Sanders will fare when Democratic or Republican rivals attack him in a high-profile fashion, which, to this point, no one has seriously done.

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Democrats face a classic collective-action problem. The party has a strong interest in publicly vetting Sanders before he becomes its nominee, but no candidate wants to be the one to go negative on him. Instead, as with Donald Trump’s Republican opponents in 2016, other Democratic candidates are seemingly hoping to pick off Sanders voters during the primary season, or at least attract their support in November, without doing the dirty work of criticizing his record. Attacks that appear to echo potential Republican talking points are especially likely to go unsaid. As a result, large numbers of voters may not learn about Sanders’s vulnerabilities and how they might be exploited in a general election until much later in the race.

The lack of scrutiny of Sanders dates back to 2016. Despite his long career in politics, Sanders was a little-known outsider before his presidential campaign against Hillary Clinton. His unexpectedly strong showing in that race made him a national figure with an unusually positive public image. Why? Few politicians ever criticized him. Sanders never seriously threatened Clinton’s hold on the nomination, so she mostly held her fire, preferring to try to keep his voters in the fold for November. Republicans largely withheld criticism as well, presumably appreciating his refusal to withdraw from the race and hoping to run against him rather than Clinton in the general election.

These attacks will come, however, if Sanders is the Democratic nominee. Any candidate will face attacks, of course, but for contenders like Sanders who have been insulated from previous criticism, the potential for damage is especially great.

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For instance, Sen. John McCain, with his reputation as a bipartisan maverick, seemed highly electable during President George W. Bush’s second term — the rare Republican whom even Democrats viewed favorably. But the polling numbers he enjoyed overstated his appeal as a party nominee. Once he took the positions required to win the 2008 GOP nomination and came under fire from Democrats, his public image became essentially that of a generic Republican, which helped enable his defeat by Barack Obama.

Similarly, Clinton had unusually strong favorability numbers entering Obama’s second term as a result of her tenure as secretary of state, which removed her from the political fray. Once she began her presidential campaign, however, attacks from Republicans and critical media coverage of controversies like her email practices and her handling of the attack on the U.S. mission in Benghazi, Libya, helped drive her unfavorable ratings back up to near-record levels.

Sanders starts from a weaker position; his candidate favorability ratings have come back to earth, declining in CNN polls, for example, from 59 percent favorable and 35 unfavorable in April 2017 to 43 percent favorable and 44 percent unfavorable in December 2019. They could decline still further given the weaknesses in his background, which are little known to wide swaths of voters. How many Americans know that Sanders is not just an avowed democratic socialist but a former supporter of the Trotskyist Socialist Workers Party, which wanted to abolish the federal defense budget and supported “solidarity” with revolutionary regimes like Iran’s and Cuba’s? Do people know that he spoke positively about Fidel Castro and the Cuban revolution (“a very profound and very deep revolution”) and even praised the Soviet Union and criticized the United States during a honeymoon trip to the U.S.S.R.? Could Sanders successfully distance himself from these statements, or would the public perceive them as disqualifying? No one knows, but the downside risk for Democrats has no precedent among front-runners in contemporary American political history.

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Moreover, though Democratic candidates don’t want to make this point in the primary race, attacks on Sanders’s praise of socialist and communist governments are likely to be especially damaging when paired with criticism of his policy proposals as big-government socialism. Even Sen. Elizabeth Warren (D-Mass.), who once assiduously sought to prevent Sanders from getting to her left, has realized the risks of Sanders’s plan to move all health care to a single-payer system and has started to edge away from the idea. Only 20 percent of voters — and just 37 percent of Democrats — say they would be enthusiastic about voting for a socialist for president.

Labels like “socialist” are abstract and poorly understood by most voters, of course; some of Sanders’s policies are indeed popular. But the penalty for extremism is real. When ideologically extreme candidates narrowly defeat moderates for a party nomination, the political scientists Andrew Hall and Daniel Thompson find, they perform even more poorly in the general election, in part because they inspire the other party’s base more than their own. For instance, former Kansas secretary of state Kris Kobach, a hard-right conservative, barely beat the Republican governor in the 2018 gubernatorial primary before losing the general election to a Democrat by five percentage points.

Trump might seem to be a counterexample, but Sanders will struggle to replicate his success. It’s true that Trump won the White House despite having unusually high unfavorable ratings and a personal background that many voters considered disqualifying. Like Trump, Sanders would surely benefit from the strong pull of party loyalty, which can help counter the doubts of some potential supporters. But Trump had a key advantage: Voters in 2016 saw him as unusually moderate, which helped him overcome those record unfavorable numbers. Though the public now sees Trump as more conservative than in the last election, it views Sanders as even more distant from the center.

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Besides his socialist positions, Sanders also has a long paper trail of writings and statements about sex, gender and race that have received relatively little attention but are likely to provoke far more controversy if he wins the nomination. In one 1969 essay, for instance, Sanders wrote that the “manner in which you bring up your daughter with regard to sexual attitudes may very well determine whether or not she will develop breast cancer, among other things.” And does his diverse coalition of young supporters know he once compared workers in Vermont to slaves?

Americans tend to hate negative ads and campaigning, but they can be informative to voters. If other candidates continue to pull their punches, Democrats may not learn more about Sanders until the whole country finds out this fall.