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The coronavirus poses an “existential threat” to President Donald Trump’s 2020 re-election, campaign insiders and GOP operatives say.

“He’s not running against Joe Biden, he’s running against a virus and the collateral damage from that,” former White House strategist Steve Bannon told The Post, saying the outbreak and its ripple effects were potentially the most complex threat ever faced by the United States. “You could have a financial crash, economic great depression and a war against a bug all simultaneously … Today is November. [Trump] is going to be weighed and measured by the American people by how [he handles] the crisis.”

One Trump campaign operative was more blunt, saying, “If he f–ks up the economic stimulus around the coronavirus, he loses.”

He continued: “Donald Trump’s biggest hurdle to winning re-election has always been the economy. If the economy is going well and cruising along, he could have sailed to a re-election. If we hit that 20% unemployment mark as the Treasury is projecting, it’s going to be hard to say that the economy is doing well.”

Polls so far offer a mixed bag. An ABC/Ipsos survey released Friday found 55% of Americans approved of Trump’s handling of the coronavirus, while just 43% disapproved — a near-total reversal from what the same poll reported a week earlier. However, the president’s overall approval ratings have slumped from post-impeachment highs of 49% to the more typical 44% he had before the outbreak according to Gallup.

On Tuesday, Treasury Secretary Steve Mnuchin raised eyebrows after warning of a possible 20% unemployment rate in the event a coronavirus economic stimulus was not passed. “I didn’t in any way say I think we are going to have that. It’s just a mathematical statement,” he clarified a day later.

Markets, however, cratered. Historic gains in the Dow Jones Industrial Average and S&P 500 Index during Trump’s tenure were erased in mere days. The closure of bars and restaurants created an instant flood of unemployment.

“Everyone takes this extremely seriously,” a member of the Trump-Pence 2020 re-election campaign advisory board told The Post. “This is a one-off act of God event. The question is what sort of long-term damage that it does. There are so many unknowns, we’ve never been here before. We haven’t had a total shutdown of the US economy before.”

After initially downplaying the seriousness of the virus, calling it a “little problem” that was “totally under control” just weeks ago, President Trump has moved aggressively to combat the spread of the disease.

‘If he f–ks up the economic stimulus around the coronavirus, he loses’

The administration shut down travel between the United States and Europe and declared a national emergency, freeing up billions in federal aid dollars to fight the plague. Trump signed a coronavirus stimulus package guaranteeing free testing and paid emergency leave on Thursday. Additional aid — including potentially $1,200 checks to every American making less than $75,000 a year and a raft of new corporate tax cuts — is in the pipeline.

“It’s a war,” Trump said during a press briefing this week, when describing how he viewed the Coronavirus fight going forward “I view it as a, in a sense, a wartime president.”

A presidential race once focused on issues like climate change, income inequality and even reparations has been reordered to reflect the nation’s singular focus on the virus.

“This will undeniably be the biggest issue in the election,” GOP strategist Luke Thompson told The Post. “His presidency sinks or sails now on his handling of this.”

Some said the virus offered an opportunity for the president to demonstrate leadership and competence and that a home-run federal response would all but guarantee a second term.

“FDR won re-election in 1936 when unemployment was 17% because people thought he was fighting for them,” GOP political consultant Ryan Girdusky told The Post.

“His argument will be is it’s me vs. the fake news, obstructionists Democrats and the China virus,” said former 2016 presidential aide Sam Nunberg. “Eventually people around him will start saying the Chinese did this to try and sink him. He’s got a great boogeyman out of this. Trump loves foils.”

“Just in general, never count out Donald Trump,” Nunberg added.

Trump handling of coronavirus crisis