By Walter Shapiro

CQ-Roll Call

If campaign consultants in both parties had their way, congressional challengers would never utter an interesting word and incumbents would have their Capitol Hill voting records airbrushed from history. Politics would be reduced to a clash between two physically attractive candidates (preferably with photogenic families), obediently reciting robotic talking points.

The major problem with this beguiling fantasy is a pesky group of human beings known as voters. Increasingly, voters crave authenticity, a hard-to-define attribute that comes across as the antithesis of poll-tested and blow-dried.

This, as much as ideology, explains the surprising rise of Bernie Sanders and the shocking presidency of Donald Trump. Both different-drummer 2016 candidates had the perfect foil in Hillary Clinton, a politician governed by caution in all matters except email servers and Goldman Sachs speeches.

All this might seem ancient history except this dynamic may again play itself out in next year’s midterms. Democratic consultants want to fight 2018 on the comfortable terrain of health care, the economy and the record of the Paul Ryan Congress. But Democratic activists will push for a far bolder agenda as they fuse the anger of the tea party movement with the mobilization tools of 2017.

What the Democratic base craves — whether they supported Clinton or Sanders in the 2016 primaries — is the same thing that an out-of-power Ronald Reagan called for during the Republican years in the wilderness after Watergate. Addressing the Young Americans for Freedom in 1975, Reagan declared that the GOP must offer “a cause to believe in … raising a banner of no pale pastels, but bold colors.”

All this brings us to last week’s British elections, which, at first glance, appear to vindicate Sanders’ boldly colored vision of a militant left-wing Democratic Party. Jeremy Corbyn — who is a throwback to the Labor Party before the moderating influence of Tony Blair — confounded the experts by leading his party to a 30-seat pickup.

The youth vote was a key to the Labor Party making its biggest percentage gain since 1945 when Clement Atlee ousted a sitting prime minister named Churchill. Not only did younger voters support Labor by lopsided margins (partly because of a promise of free college tuition), but the turnout rate among those ages 18 to 24 also rose from an estimated 43 percent in 2015 to 66 percent this time out.

None of this was lost on Sanders who eagerly embraced Corbyn, his trans-Atlantic soul mate. Speaking Saturday night to activists in Chicago, Sanders invoked Labor’s symbolic victory: “They won those seats, not by moving to the right, not by becoming conciliatory — they won those seats by standing (up) to the ruling class of the U.K.”

But there is a danger in over-interpreting the British results. Nothing in American political life equals the shock waves from Brexit — a withdrawal from Europe that will deeply affect the British economy, immigration and the ability to work and travel abroad.

Also, ideology matters more in a parliamentary system where party manifestos can be enacted into law without worrying about filibusters and the equivalent of the Freedom Caucus. And while Labor was aided by organizers who worked for Sanders in 2016, get-out-the-vote drives and voter targeting have traditionally not been as advanced in Britain as in America.

Perhaps the biggest trans-Atlantic difference is that while Corbyn was steeped in the Labor Party, Sanders continues to exude a high-minded disdain for the Democrats. As Sanders put it in his Saturday night speech at the People’s Summit: “The Democratic Party cannot continue to be a party of the East Coast and the West Coast. It must be a party of all 50 states. Unfortunately, to a large degree, the Democratic leadership has ignored the needs of working people.”

There is, to be sure, an increasing culture gap between Central Park West and Cedar Rapids. But with the decline of the labor movement, the Democrats, beginning in the 1980s, began to turn to investment bankers in New York and the movie community in Hollywood as the party’s primary sources of fundraising. Throw into the mix the fact that Democrats have long viewed themselves as the party of high SAT scores — and you have a formula for the death of the last remnants of FDR’s New Deal coalition.

In theory, a mobilized base of activists can offer the Democrats an alternative source of campaign funds. But, up to now, online donors have only demonstrated their collective clout in high-profile races like Democrat Jon Ossoff’s House campaign in Georgia’s 6th District. While Ossoff has raised a record-shattering $23 million, it would require more than $1 billion donated online to pull off the same trick in every contested House race in 2018.

So even as they decry Wall Street and West Coast elites, Democratic congressional candidates will make the familiar pilgrimages to New York and L.A. as they prepare for 2018. It is an unalterable truth of politics that it is hard to wage class warfare from Park Avenue living rooms.

With congressional Republicans cowering in the face of Trump’s violations of democratic norms, the 2018 battle to control the House may be the most important set of off-year elections in modern memory. The danger is that Democratic fratricide could prove as destructive to the party’s hopes as it was during the Vietnam years.

Rather than refighting the 2016 primaries, the challenge facing the Democrats is how to assemble the broadest possible anti-Trump coalition while respecting the passion of the voters for authenticity.

Roll Call columnist Walter Shapiro is a veteran of Politics Daily, USA Today, Time, Newsweek and The Washington Post.