Sen. Bernie Sanders (I-Vt.) addresses a rally against the Republican tax plan outside the Capitol on Nov. 1. (Chip Somodevilla/Getty Images)

James Kirchick is a visiting fellow at the Brookings Institution and author of “The End of Europe: Dictators, Demagogues, and the Coming Dark Age.”

When Russia interferes in the 2020 presidential election on behalf of Democratic nominee Sen. Bernie Sanders (I-Vt.), how will liberals respond?

That Moscow favored Donald Trump over Hillary Clinton should be obvious to all but the most hardened of Republican partisans. The clearest evidence was that Russian operatives hacked and leaked Democratic Party email accounts to undermine its nominee. And any casual observer of Russian disinformation outlets could have easily discerned what a U.S. Intelligence Community report characterized as the Kremlin’s “clear preference” for Trump.

But the past few weeks have produced a more sophisticated picture of the Kremlin’s destabilization effort. We’ve learned that it’s bipartisan, post-ideological and ongoing. Republican Sen. Charles E. Grassley (R-Iowa) had it exactly right when he said at a recent Judiciary Committee hearing: “Russia does not have loyalty to a political party in the United States. Their goal is to divide us and discredit our democracy.

Consider the Facebook group BlackMattersUS, one of 470 reportedly Kremlin-run pages and accounts the social network shut down since the election. A little over a week after Trump won the presidency, the site promoted a rally against the very incoming commander in chief Moscow had supported. Some Russian-created social media accounts targeted LGBT activists and environmentalists; others successfully organized protests across the country on either side of the Black Lives/Blue Lives Matter divide.

Instigating turmoil within its adversaries has been a Russian ambition since well before the Bolshevik Revolution 100 years ago. Russian subversion of American democracy reached a peak during the Cold War, but it didn’t end with the collapse of Soviet communism. “It is especially important to introduce geopolitical disorder into internal American activity, encouraging all kinds of separatism and ethnic, social and racial conflicts, actively supporting all dissident movements — extremist, racist and sectarian groups, thus destabilizing internal political processes in the U.S.,” Russian nationalist ideologue Alexander Dugin counseled in 1997. “It would also make sense simultaneously to support isolationist tendencies in American politics.”

Across Europe today, Russian political meddling is ideologically promiscuous, assisting not only British Brexiteers and France’s National Front but also German Social Democrats and Greek post-communists. On the television channel formerly known as Russia Today (RT), guests range from far-right Holocaust deniers to far-left supporters of Syrian dictator Bashar al-Assad. Reminiscing on the heyday of Occupy Wall Street, movement co-founder Micah White recalled that the network’s many favorable stories were “obvious attempts to co-opt our social protest by amplifying it and becoming the movement’s primary mouthpiece and media source.” For all their diversity, these Russian interventions follow the same general design: deepen societal divisions, popularize conspiracy theories and discredit America abroad.

Russia has been playing this polarizing game for decades. And by delegitimizing the country’s first black president, speculating that the election would be “rigged,” and denigrating bedrock elements of the political West such as NATO and the European Union, Trump wittingly served all three Russian objectives on the campaign trail — and continues to do so in office.

Yet from Moscow’s perspective, it’s entirely possible a left-wing Democrat will better serve its purposes in the future.

Consider that Russia’s first major foray into the 2016 campaign was intended to help not Trump but Sanders. On the eve of last year’s Democratic National Convention, WikiLeaks released a tranche of emails stolen from the Democratic National Committee purporting to show institutional favoritism for Hillary Clinton. Just as Republicans would unscrupulously tout Clinton campaign chairman John Podesta’s hacked emails, Sanders supporters showed no compunction exploiting material stolen by Russian operatives and selectively published by WikiLeaks. DNC chairwoman and Clinton ally Rep. Debbie Wasserman Schultz (Fla.) was forced to resign, and Sanders delegates booed convention proceedings throughout the week.

If there’s anything we ought to have learned from last year’s campaign, it’s that a lot can happen in three years. Few imagined that the party whose previous presidential nominee called Russia our “No. 1 geopolitical foe” would, at the next election, select a candidate who hailed Russian President Vladimir Putin as a “great leader.” The Kremlin ultimately favoredc Trump over Clinton because it preferred his nationalist populism to her liberal internationalism. Had Sanders won last year’s Democratic primary and faced off against Jeb Bush, it’s probable the Russians would have (again) intervened on behalf of the man who honeymooned in the Soviet Union, advocates massive cuts in defense spending and, like Trump, calls the American system “rigged.”

Messrs. Trump and Putin could also have a personal falling out, or a geopolitical crisis might erupt, putting the United States and Russia at odds. All this might create the conditions for the emergence of a Democratic peace candidate more to the Kremlin’s liking than the unpredictable and militaristic Trump. And coming to the aid of the American left would hardly be out of character for Moscow.

Last year, the vast majority of Republicans looked askance at Russia’s interference because it worked to the benefit of their presidential nominee. American politicians should follow the example of their German counterparts, who — after a 2015 cyberattack on the Bundestag attributed to Russia and fearing the sort of influence operation conducted in the United States — brokered a “gentlemen’s agreement” not to exploit material hacked and leaked by foreign actors in the run-up to September’s federal election.

Russia’s potential meddling on the side of Democrats will present a test for our newborn liberal cold warriors. Will they denounce such unsolicited comradely assistance? Or find ways to excuse it, as so many Republicans did and continue to do?