In 2016, Democrats mistakenly followed assurances by pollsters, pundits, and consultants who said that they could cruise to an easy victory over Donald Trump by playing it safe in the general election, after their bruising primaries. The sad denouement for Democrats and the nation is history. Remarkably, Democrats are in danger again of falling prey to this year’s conventional wisdom that they can win back the presidency in 2020 by playing not to lose, this time through avoiding another spirited primary contest.

Politico reported on Tuesday that “interviews with nearly 20 Democratic elected officials, party chiefs, labor leaders and operatives the past week revealed an air of foreboding verging on alarm that the debates will degenerate into a two-night, bare-knuckle brawl” resulting in a clichéd “circular firing squad.” These insiders have “the divisive 2016 Democratic primary fresh in their minds” and with “the current presidential candidates starting to take swipes at one another, the fear is that voters will be left with the impression of a bickering, small-minded opposition party.”

My advice to Democratic presidential hopefuls in the upcoming debates is to ignore the conventional wisdom and stop worrying. Go ahead and have unconstrained debates without fear of negative consequences for the general election campaign in 2020. Recent history proves that tough primary struggles only hurt the party holding the White House, not the challenging party. The brawl between Hillary Clinton and Bernie Sanders damaged the Democrats in 2016 only because they were the incumbent White House party.

Since 1968, no party in charge of the White House has survived a bitter contest for its presidential nomination, whether or not the sitting president is up for reelection. During this fifty-year span, the incumbent party has forfeited the presidency after nomination battles between several Democrats in 1968, Republicans Gerald Ford and Ronald Reagan in 1976, Democrats Jimmy Carter and Ted Kennedy in 1980, and a host of Republican candidates in 2008. For example, Ford, whose nomination followed fellow Republican Richard Nixon’s resignation, ran an ad saying, “Governor Reagan couldn’t start a war, President Reagan could.” Kennedy, who challenged the Democratic party’s sitting incumbent, blamed Carter for the taking of American hostages in Iran, saying, “Had he made different decisions … our diplomats would still be going about their business in Tehran.”

But it’s a different story for the out-of-power party. The 2016 primary contest within the challenging Republican Party could hardly have been scripted as more bitter or divisive. Donald Trump denigrated his opponents with nicknames like “Lyin” Ted Cruz and “Low Energy” Jeb Bush. Trump’s opponents, in turn, assailed him as unqualified for the presidency, lacking in Jeb Bush words, “temperament or strength of character” or “respect for the constitution.” Yet the clashing Republicans won back the presidency in 2016.

So too did challenging parties after difficult primary struggles between Democrats Barack Obama and Hillary Clinton in 2008 and George W. Bush and John McCain in 2000. “Shame on you, Barack Obama,” Clinton said after Obama belittled her policies. “It is time you ran a campaign consistent with your messages in public.” Obama, in turn, charged in a radio ad that "Hillary Clinton will say anything to get elected.” During the 2000 primary campaign in South Carolina, Bush supporters circulated bogus stories that McCain had fathered a “Negro child” out of wedlock. McCain countered that Bush “twists the truth like [Bill] Clinton, we’re all pretty tired of that.” Both Obama and Bush won. By contrast, challenging Democrat John Kerry in 2004 and challenging Republican Mitt Romney in 2012 lost their general elections after relatively less contentious nomination campaigns.