For a would-be president, JD Vance has an unfortunate habit of getting into fights he cannot win. Three losing battles in the past week – with Iranian negotiators, Hungarian voters and Pope Leo – brought censure, humiliation and mockery raining down on his head. None were of Vance’s choosing. All were fought vicariously on Donald Trump’s behalf.

The vice-president is paying a high price for sycophantic loyalty to his boss. His poll ratings are plunging. His Maga succession hopes falter. He suffers by association – although his own inflammatory statements and misjudgments often make matters worse. Yet amid growing doubts about Trump’s mental health and fitness to govern, Vance remains the White House’s next-in-line.

Given he’s expected to seek the presidency in 2028, the question arises: will Vance continue to meekly act as Trump fall guy for another bruising two-and-a-half years and hope to survive, as Claudius survived Caligula? Or, faced by career-ending disaster as the “mad king” drags everyone down, will the worm turn, as Brutus turned against Caesar?

Vance’s loyalty is not reciprocated. Remember the fate of Trump’s first vice-president, Mike Pence, who refused to block the 2020 election result. Trump reportedly backed Capitol Hill rioters who wanted Pence hanged for treason. If the mood takes him, Trump will happily throw faithful followers under the bus, no matter how they bow and scrape. Ask Pam Bondi. Yet Vance’s loyalties and beliefs are flexible, too. Until he jumped on the Maga bandwagon, he was a fierce critic of Trump, warning he could become “America’s Hitler”. He reinvented himself as an immigration hawk, defending Trump’s infamous 2024 campaign lie that Haitian migrants stole and ate people’s pets.

Vance once passionately opposed overseas military adventurism. In office, he has supported attacks on Venezuela, Iraq, Syria, Yemen, Somalia, Nigeria and now Iran. He converted to Catholicism in 2019 and has since used it to widen his appeal to religious conservatives. Yet he frequently, arrogantly challenges the church’s authority and papal teachings.

In short, Vance is a crude opportunist, a self-proclaimed hillbilly peddling political moonshine. But his position is stronger than other cabinet members in one key respect. Trump cannot fire an elected vice-president – though under the 25th amendment to the constitution, Vance could help fire Trump. A group of Democrats in Congress wants him to join a special commission that would do exactly that if Trump were judged unfit.

Vance will bide his time. But Trump’s Iran fiasco and increasingly unhinged behaviour is eroding grassroots support – and that hurts his deputy, too. Vance’s lead over the secretary of state, Marco Rubio, in the notional contest for the Republican nomination is slipping. Among the wider public, according to one poll, he is the most unpopular VP in modern history. Big Democrat gains in November’s midterm elections could see him (and many other Republicans) turn on Trump.

Vance has been triply screwed over during the past week. Viktor Orbán, heading for defeat in Hungary’s election, wanted the president to stump for him. But Trump abhors losers, so he sent Vance instead. This thankless mission made no difference to the outcome, which delivered a body blow to the European ethno-nationalist Christian right backed by Trump – and by Vance in his infamous February 2025 Munich security conference speech. Vance is now closely identified with this consequential defeat.

Likewise, when Trump – desperately trying to talk his way out of his Iran disaster – needed someone to shoulder the risk, he picked on hapless Vance. Last weekend’s negotiations in Islamabad were never going to succeed in one day, because Trump lacks an understanding of Iranian resilience and because brute force has not achieved any of his basic aims. Yet the talks also flopped because Vance, with zero experience as a high-stakes negotiator and zero personal knowledge of Iran, had no authority to cut a deal. He was obliged to constantly check back with Trump by phone. Vance had feebly opposed the decision to go to war, then gone along. Now he is the public face of Trump’s failure to impose instant peace.

Trump’s ongoing, offensive attacks on Pope Leo are, perhaps, even more deadly for Vance’s standing and reputation. As the administration’s most senior Catholic, he might have been expected to defend the Holy Father. Trump’s puerile insults and blasphemies have generated a huge backlash among outraged Christians in the US, Italy and across the world.

Instead, revealing a stunning lack of political savvy, Vance questioned Leo’s truthfulness. He told the pope to stop expressing pro-peace sentiments and “stick to matters of morality”. This was crass. Suggesting war and peace is not a moral choice is as foolish as maintaining that sport and politics are separate. (For proof, just watch Trump exploit this summer’s men’s football World Cup, co-hosted by the US).

Trapped in self-harming subservience to his master, Vance hadn’t finished alienating core religious voters. He claimed a sickening image posted by Trump, in which the president was depicted as a Jesus Christ figure with divine light coming from his hands, was just a “joke”. This is the standard excuse of bullies the world over when obnoxious talk is called out. And Leo, in denouncing the global ravages of a “handful of tyrants”, has since called out Vance and his boss big time. Once again, the veep is on a hiding to nothing.

At the close of a tumultuous week, it’s plain that Trump is sliding ever closer to the personal and political precipice as waves of public dissatisfaction swell into a flood. To say he’s gone “crazy” is a subjective, non-clinical judgment, but it’s one prominent voices on the left and the divided Maga right are now vociferously making. Even if the president is fit as a flea, as he insists, he doesn’t appear so to most Americans. Polls show they think he’s too old, that he lacks sound judgment, that he’s erratic.

As Trump rants, raves, flails and sinks, Vance can reposition himself as a Maga true believer, as the natural heir – or, at least, as a leader whose sanity is not in doubt. He has an ever lengthening list of reasons to break ranks, distance himself, and challenge the ageing, raging King Lear-like president – if he can summon sufficient wit and courage. Yet Vance has demonstrated convincingly in recent days that he, too, is currently unfit for the top job. He comes across in public as weak, uninformed, angry, insecure and easily manipulated.

As a professed Catholic, Vance ostensibly believes in the forgiveness of sins. If, between now and November 2028, he can learn statesmanship, practice humility and moderation, and heed what Abraham Lincoln called the “better angels of our nature”, he may yet find redemption. At 41, Vance still has time to reinvent himself again. But it will take a miracle.

• Simon Tisdall is a Guardian foreign affairs commentator