In politics, it helps to have an enemy.

The Soviet Union served that function for American presidents of both parties during the Cold War. Radical Islamic terrorism and al Qaeda played the role for George W. Bush, ISIS and illegal immigrants for Donald Trump in 2016. Democrats from Barack Obama to Bernie Sanders and Elizabeth Warren have run against Wall Street “fat cats,” “millionaires and billionaires,” and what they call big pharma and big oil.

To understand the 2020 presidential campaign shaping up in America, it helps to look at the enemies each campaign has started to define. Given the frequently made claim that America is more polarized than ever, it’s surprising, and maybe even encouraging, that there’s an emerging consensus on two of the top enemies — the coronavirus and China.

President Trump began his Mount Rushmore speech by thanking “the doctors, nurses, and scientists working tirelessly to kill the virus.” In a July 5 tweet, Mr. Trump called it the “China virus.”

A June 22 statement from Vice President Biden’s deputy campaign manager, Kate Bedingfield, previewed the Biden campaign’s messaging on these issues: “Trump failed us from the very beginning of this crisis by refusing to take the coronavirus seriously — ignoring warnings from Vice President Biden and his own officials about the threat of COVID-19 as it spread, choosing instead to mindlessly parrot the Chinese government’s propaganda even as they ran the table on us in trade negotiations.”

Said Mr. Biden at a June 4 fundraiser, “I’m the guy who told the CDC months ago that we should keep meticulous records on who is contracting Covid as well as who is dying.”

An argument between Messrs. Biden and Trump over who has the best strategy and tactics and ability to deliver in combating the twin foes of China and the coronavirus will make for a hard-fought and vigorous campaign. The mere fact that they can agree that those are two main challenges, though, may be somewhat reassuring that America isn’t as badly divided as some claim.

Where Messrs. Biden and Trump disagree is on the identity of the enemy within. There, the unity falls apart.

According to Mr. Biden, the main threat is white supremacists and neo-Nazis. During the primaries, Mr. Biden constantly talked about the August 2017 white nationalist rally in Charlottesville, Va., and Mr. Trump’s response to it. Mr. Biden asserted that “the threat to this nation was unlike any I had ever seen in my lifetime.”

Mr. Trump, for his part, depicts the main threat — a “growing danger,” as he called it in the Mount Rushmore speech — as “Angry mobs…trying to tear down statues of our Founders, deface our most sacred memorials, and unleash a wave of violent crime in our cities.”

Mr. Trump described it as “a new far-left fascism,” a “left-wing cultural revolution…designed to overthrow the American Revolution.” In a July 5 speech at the White House, the President spoke of “defeating the radical left, the Marxists, the anarchists, the agitators, the looters.”

The Trump camp sees this as a decisive issue in the coming election. The executive director of the Republican Jewish Coalition, Matt Brooks, tweeted a video clip of protesters outside the White House setting an American flag on fire on July 4 while chanting “America was never great.” Mr. Brooks commented, “And this is why @realDonaldTrump will win election in November.” He could be right.

The risk for Mr. Trump’s re-election chances, though, is that rather than seeing him as the last line of defense preventing America from being taken over by the flag-burning mob, voters will blame Mr. Trump for creating the conditions that produced the mob. This is Mr. Biden’s critique — that Mr. Trump has divided America.

For Mr. Biden, “The mob has arisen on Trump’s watch” is a complicated message, because many of his supporters and activists see the people in the streets not as a mob but as a primarily praiseworthy group advocating peacefully for racial justice and against police brutality.

If that’s so, one could even argue that Mr. Trump should get credit for raising racial consciousness and sensitivity to previously unseen levels. Some version of blaming Mr. Trump for the civil unrest, though, may yet help Mr. Biden convince swing voters in swing states to add Mr. Trump himself to the list of America’s political enemies in 2020.

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Image: Drawing — ‘Archers Aiming at a Herm,’ after Michelangelo — by Elliott Banfield, courtesy of the artist.