Joe Biden delivered a deeply personal rebuke of Donald Trump while marking his first joint appearance with Hillary Clinton on the campaign trail on Monday, invoking the military service of his late son Beau to cast the Republican nominee as “totally, thoroughly unqualified” to serve as president.

“No major party nominee in the history of the United States of America ... has known less or been less prepared to deal with our national security than Donald Trump,” Biden said in his hometown of Scranton, Pennsylvania, while sharing the stage with Clinton.

Recalling the deployment to Iraq of his son Beau, who died of cancer last year, the vice-president added: “Had Donald Trump been president, I would have thrown my body in front of him … to keep him from going if the judgment was based on Trump’s decision.”

While Biden has been an outspoken critic of Trump’s rhetoric against immigrants and Muslims, this was his most scathing takedown yet of the Republican nominee’s fitness to assume the role of commander-in-chief. Mocking Trump’s public affinity toward Russian president Vladimir Putin, Biden tore into the bombastic mogul as someone who “would have loved Stalin”.

“This guy’s shame has no limits,” Biden said. “He’s even gone so far as to ask Putin and Russia to conduct cyberattacks against the United States of America.”

“Even if he is joking – which he’s not – what an outrageous thing to say.”

Biden, who flirted with a third run for the presidency last year, returns to the stump without a place on the ticket for the first time in eight years. But he is expected to be no less a powerful surrogate for Clinton, particularly among the blue-collar voters comprising America’s ‘rust belt’ and upon whom Trump’s narrow path to the presidency is predicated.

In his trademark emotive tone, Biden repeatedly hushed a crowd of more than 3,000 who packed into a sports facility to celebrate their native son’s return and hoisted signs reading “Welcome home”. Lowering his voice, the vice-president urged voters not simply to cheer but to fully grasp how Trump’s persona ran counter to the working-class experience the businessman claims to champion.

“What bothers me most about Donald Trump is his cynicism is unbounded,” Biden said, before invoking the former reality TV star’s “You’re fired” catchphrase.

“Think about everything you learned as a child,” Biden said, turning a full circle from the podium to look directly at his audience from all corners of the lectern, “no matter where you were raised.”

“How can there be pleasure in division and ‘you’re fired’?”

Biden was originally scheduled to make his debut on the campaign trail last month at a rally that was postponed due to the fatal shooting of five police officers in Dallas. On Monday, the vice-president echoed many of the themes from the impassioned speech he gave on Clinton’s behalf at the Democratic national convention in Philadelphia, dubbing once again the notion that Trump represented American workers as “a bunch of malarky”.

“This guy doesn’t care about the middle class,” Biden said. “He doesn’t understand it. He doesn’t have a clue.”

Few campaigners are known to possess Biden’s congenial, free-wheeling speaking style in an era marked by the widespread sentiment that Washington is out of touch with everyday Americans. As the son of a used car salesman, Biden has weaved his personal story into the political narrative with such repetitiveness over the last few decades that it would border on redundancy coming from any rank-and-file politician.

But Biden shared the vignettes from his childhood that have long been a staple of his stump speeches with the same vigor as if told for the first time. There was the image, for example, of Biden finding his harried father pacing back-and-forth at work – “ashamed” to tell his son he had been turned down a loan to send him to school.

“Hillary understands that the most damaging thing to a parent is to look at a talented child or a sick child in need and not be able to do anything,” Biden said, pointing to her support for college affordability and expanding access to healthcare.

“She’s always been there, that’s her life story. Let’s state the obvious: That is not Donald Trump’s life story.”

It was a homecoming not just for Biden but also for Clinton, whose father was born in Scranton and worked at a lace mill. The Democratic nominee took aim at her opponent’s tax agenda, coining as “the Trump loophole” plans she said would benefit only the nation’s top earners.

Clinton countered Trump’s economic message by borrowing a phrase of Biden’s as he sat behind her during her own remarks: “Don’t tell me what you value. Show me your budget and I’ll tell you what you value.”

“Sometimes he says he won’t tell anyone what he’ll do because he wants to keep his plan, quote, secret,” Clinton added of Trump. “And then it turns out the secret is, he has no plan.”

The former secretary of state also hinted that Biden may have a role in her administration, should she be elected, in continuing his work at the helm of the Cancer Moonshot initiative announced by Barack Obama during his State of the Union address this year. The president placed Biden in charge of the federal effort to invest in cancer research and expand access to treatment, after his son Beau died in May 2015.

“This is personal to Joe Biden. He knows it’s personal to a lot of people,” Clinton said.

Clinton currently holds a double-digit advantage to Trump in Pennsylvania, a reliably blue state that Trump hopes to swing in his favor by seizing on economic anxiety. Biden is expected to focus his time in the state and other blue collar battlegrounds, such as Ohio and Michigan.

The potential impact of his persuasion was readily apparent, as the most common reaction from attendees leaving the rally was that Biden “spoke from the heart”.

Tom Dowling, a union member of the International Brotherhood of Electrical Workers, drove from New York with his children to witness the Biden-Clinton tag team in the so-called “Electric City”.

“He seems like a genuine, caring, loving person,” said Dowling, who fought back tears while highlighting Biden’s relationship with his late son.

Jason Sharockman, a native of nearby Pottsville, said Biden was “very influential to the north-east”.

“What can you say? He gives a great speech.”