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Last week was a triumphant one for Joe Biden. The former vice president secured a fulsome endorsement from Sen. Bernie Sanders, I-Vt., his chief rival for the Democratic nomination. The next day, Barack Obama gave his endorsement to Biden, and the Democrats seemed to be uniting behind their nominee.

Despite the victorious tone, there were notes of concern. A number of voices on the left started trying to give Biden advice. And all of that advice dealt with the idea, as one headline stated, that “Joe Biden has a big problem with the youth vote.”

At Jacobin, Branko Marcetic, the author of a book subtitled “The Case Against Joe Biden,” offered “free advice” to on how to get “young, left-wing voters on board.” A coalition of eight interest groups focused on “building political power for young people” sent Biden an open letter advising him on how to get “more young people enthusiastically supporting ... you.”

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And Rep. Alexandria Ocasio-Cortez, D-N.Y., gave an interview with the New York Times in which she said Biden needed to “get uncomfortable” with his issue stands in order to reverse his “underperformance among Latinos and young people.”

The root of the concern for these figures to Biden’s left is that he will not generate sufficient turnout among young voters in November to propel him past Donald Trump. Based on results in the Democratic primary, this concern is justified.

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Our Fox News Voter Analysis of Super Tuesday voters found that Biden won only 15 percent of 18- to 29-year-olds that day, and 21 percent of 30- to 39-year-old Democrats. In the primaries held after Super Tuesday, Biden trailed Sanders among 18- to 29-year-olds and 30- to 39-year-olds in every state that we polled.

But like they say in ads for financial products: past performance does not necessarily predict future results.

A review of recent elections shows that just because a candidate has a weakness with a particular group of primary voters does not mean he will be weak with that same group in the general election.

The warnings that Democrats have offered Biden in 2020 echo those that Obama received in 2008 from Hispanic Democrats that “he risks losing the Latino vote in the general election if he does not reach out to Hispanic voters.” These warnings were overstated; in November, Obama won Hispanics in a landslide 67-32, a 15-point improvement over John Kerry’s performance in 2004 with this group.

What explains why a candidate can fare poorly with a group in the primaries and win them by a landslide in the general?

Trump faced a similar problem in 2016 with Republican voters who attended religious services at least once a week. The Pew Research Center found that “[t]wo-thirds of regular churchgoing Republicans were not supporting Trump for the GOP nomination even in April.” Yet in the general election, Trump won white evangelicals by a whopping 81-16 margin, three points better than Mitt Romney did with this group in 2012.

Romney had his own weaknesses in the 2012 primary. According to one analysis,“ the demographic breakdown between Romney and Rick Santorum in the primaries was remarkably consistent ... Santorum won among evangelicals, tea party supporters and those who described themselves as the most conservative voters.” Come November, Romney easily won conservative voters 82-17, a four-point improvement on John McCain’s performance with conservatives in 2008.

So what explains why a candidate can fare poorly with a group in the primaries and win them by a landslide in the general?

First, the November electorate is vastly different from the electorate in the primary. In 2016, 31 million votes were cast in the Republican primary, In November that year, over a 100 million more people cast their votes. The candidate may be the same but the voters surely are not.

Also, the mix of candidates is different in the general election, and voters compare not multiple Democrats or Republicans to each other, but a Democrat to a Republican. And in a general election, the performance of the incumbent party in office is often the most important factor in explaining shifts in vote choice. Hispanics in 2008, conservatives in 2012, and working-class whites in 2016 were all displeased with how the policies of the Bush and Obama administrations had affected their group.

So how will Biden perform among young voters in 2020? As of now, we can only speculate. The premise behind the left-wing suggestion that Biden make policy changes to appeal to young voters is to avoid what happened in 2016. Hillary Clinton proved unable to get enough young voters to the polls to overcome Trump’s advantages with evangelicals and other working-class whites.

Yet, the 2018 midterms tell us a different story. Just as they did last week, progressive Democrats urged their party to shift their platform to the left to appeal to non-voters in 2016, pushing “Medicare-for-all” as an issue that could boost turnout. Justice Democrats, one of the signers of the letter to Biden last week, issued a detailed report to make this case.

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The party broadly ignored this advice, instead focusing their health care appeals on small-bore issues like protecting the ban on insurance companies rejecting applicants for preexisting conditions. The Democrats' turnout strategy in 2018 was to rely on anger toward Trump to motivate voters. This worked. Democrats took back the majority in the U.S. House in large part by winning young voters 64-34. Midterm turnout reached unprecedented levels.

Joe Biden might do well to follow this model from 2018 and hew to the center of the ideological spectrum and let Donald Trump be in charge of his voter mobilization program.

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