To the political right in America, Joe Biden’s son Hunter has been the gift that keeps on giving, with his public struggles with addiction, scandalous private life and tangled business life. To the left, Hunter’s travails are dismissed as a Republican political obsession and a talking point for tabloid journalism and internet gossip.

But last week, two witnesses called before a federal grand jury seated in Wilmington, Delaware, which is looking into the tax affairs of the president’s son, made the subject harder to avoid.

First there was Lunden Roberts, with whom Biden has a three-year-old unacknowledged child. Then Zoe Kestan, an ex-girlfriend and lingerie and textile designer, spent five hours giving testimony on Biden’s spending, including – reportedly – stays at the Chateau Marmont in Los Angeles, where, in 2018, allegedly, Biden was preoccupied with cooking crack cocaine.

Wretched and salacious as that sounds, much of Hunter Biden’s story, detailed at length in his autobiography Beautiful Things, published last year, tends that way. “I’m not a curio or a sideshow to a moment in history,” Biden claimed in his book. “I’ve worked for someone other than my father, [I] rose and fell on my own.”

But that’s not how Joe Biden’s political enemies see it.

Donald Trump tried to make issue out of Hunter’s business dealings in Ukraine, Russia and China, which included high paid consultancies and gifts, and allegations that, as vice-president, Joe Biden had shaped American foreign policy in Ukraine to benefit his son.

On the political flip-side, House intelligence committee chair Adam Schiff said the laptop was a “smear” from Russian intelligence, and 50 former intelligence officials said it was probably Russian disinformation. Now, however, almost no one disputes its authenticity.

Hunter Biden confirmed that he was under federal investigation over a tax matter in December 2020, days after his father was elected president. Attorney general Bill Barr said he had “not seen a reason” to appoint a special counsel to oversee investigations, which include an investigation by a federal securities fraud unit in New York and another in Pennsylvania.

Biden has not been charged with any crime, and David Weiss, US Attorney for Delaware who oversees the inquiry, is regarded as a straight-shooter unlikely to be swayed by political pressure. He was appointed by Trump on the recommendation of the state’s two Democratic senators and has not been replaced by Joe Biden.

Weiss, according to Politico, avoided taking any decisions that would alert the public to the existence of the inquiry before the 2020 presidential election – and a repeat of the FBI’s Hillary Clinton missing emails investigation, which may have influenced the outcome of the 2016 contest.

But the larger question – beyond whether Hunter Biden correctly met tax obligations during a period in which, by his own telling, he was being paid $50,000 a month by Ukrainian firm Burisma – are Biden’s financial ties to foreign figures and businesses while his father served as Barack Obama’s No 2.

Illegal lobbying is an issue that shadowed Trump throughout his presidency, leading to the conviction of Paul Manafort, Trump’s 2016 campaign manager, on tax fraud charges. Manafort later pleaded guilty to violating the Foreign Agents Registration Act (Fara) by providing false statements, laundering money, witness tampering and failing to register as an agent of the Ukrainian government.

Last year, Thomas Barrack, a friend and former adviser to Trump, was arrested on charges that he and others failed to inform the US government that they were working to influence US foreign policy on behalf of the United Arab Emirates.

Under US statutes, all persons acting politically or quasi-politically on behalf of foreign entities in the US must properly disclose their activities.

In addition to Hunter Biden’s ties to Ukraine through the gas company Burisma, he has sat on the boards of BHR Partners, a private investment fund backed by a number of Chinese state entities; a hedge fund, Paradigm; a consultancy, Seneca Global Advisors; and the fundraising firm Rosemont Seneca.

Republicans, including the senior Iowa senator Chuck Grassley, have called on the justice department to evaluate whether Hunter or Joe Biden’s brother James Biden should have registered as foreign agents over their business arrangements with the Chinese government-backed energy company CEFC.

In 2018, Business Insider reported that Hunter Biden sought an annual $2m retainer to aid in the recovery of Libyan assets frozen by the Obama administration during Muammar Gaddafi’s rule. The list of accusations goes on.

According to Jonathan Turley, a legal scholar at the George Washington University Law School, “influence-peddling is a virtual spectator sport in the nation’s capital – a protected corruption”.

Turley said: “It’s how powerful ruling elites make much of their money, and Congress has never seriously tried to crack down on it. The children and spouses of powerful leaders continue to receive windfall payments from companies and foreign interests, but we’ve never quite seen the likes of Hunter Biden’s enterprises. His contracts go beyond anything we’ve seen before.”

Joe Biden has long insisted that his son did nothing wrong. “There’s nobody that’s indicated there’s a single solitary thing that he did that was inappropriate, wrong … or anything other than the appearance,” Biden said two years ago.

But should the Delaware panel recommend criminal charges, it could ricochet around the second half of his father’s administration.