The Trump campaign’s secret weapon against Joe Biden will be Bernie Bros.

“Bernie Sanders supporters have been laying the groundwork for many of the lines of attack that the Trump campaign will be using against Joe Biden in the general,” a senior Trump campaign insider told The Post. “From his serial dishonesty, to his obvious issues, to his attempts to cut Social Security. We are going to have a field day.”

During the 2020 primaries, nobody’s knives for Biden were sharper than Sanders’ rabid supporters online and in far-left media.

Echoing critiques they made against Hillary Clinton in 2016, the Bernie Bros laced into Biden for being insufficiently left on a number of their core issues: healthcare, which they say would leave 10 million Americans uninsured as opposed to Sanders’ “Medicare for All”; Biden’s vote for the Iraq War in 2003, and his chumminess with billionaires and SuperPacs.

In an effort to court the Bros, the Biden campaign announced this week that he would call for lowering medicare eligibility to 60 and canceling student loan debt for some poor and middle-income families. The effort, however, was swiftly denounced as woefully insufficient by the Sandernistas — including his own former National Press Secretary Briahna Joy Gray.

Other criticism goes for the jugular.

“There’s much talk online, especially among Bernie Sanders supporters, about whether Joe Biden is in cognitive decline, whether he has dementia, why he’s so incoherent,” Al Jazeera pundit and Sanders uberfan Mehdi Hasan said in March 9 video. “Now I’m no doctor. I’m not going to make a medical diagnosis, apart from to say that in terms of his speech and his memory, there has clearly been a lot of decline in recent years.”

The video — published nearly a week after Sanders’ collapse on Super Tuesday — has been retweeted more than 6,000 times and racked up more than a million views in five weeks.

“We have a running tab here at Trump campaign headquarters of Mehdi Hassan’s many in-kind contributions,” laughed the campaign insider, saying Team Trump — which assiduously avoids using the D-word when talking about Biden — would never have put out something as audacious as Hasan’s video.

With Sanders’ eventual exit all but a foregone conclusion for weeks, there has been much handwringing within the Democratic Party over preventing “Broxit” — an exodus of Sanders’ most devout fans.

The threats and warnings from Sanders supporters throughout the primary are no idle threat. In 2016, 12% of them voted for Trump, according to an analysis by Cooperative Congressional Election Study. All told, 216,000 Bernie voters switched sides in Michigan, Pennsylvania and Wisconsin — far exceeding Trump’s 77,744 vote combined margin of victory in those swing states.

There were also untold numbers of Sanders die-hards who stayed home or bolted to the Green Party. Among them was Gray, who publicly admitted to voting for Jill Stein over Clinton in 2016.

The Bros mostly shrug off any responsibility for future Trump attack ads against the Democratic nominee.

“That’s the point of a primary and I think anyone treating legitimate and fair criticism during a primary as helping the opponent in the general election is disingenuous,” said Jordan Uhl, a progressive activist.