Not many US allies welcomed Donald Trump’s presidency. The Gulf states did. Their leaders condemned the Obama administration as soft on Iran. The Obama administration had drawn red lines in Syria, and then failed to react when the red lines were over-run.

No Arab country would have loved Mr Trump’s call for a ban on Muslim travel into the United States. But as a Gulf leader once said to me: “You cannot live in this neighbourhood if you don’t know how to walk on both sides of the street at the same time.” Washington’s allies in the Gulf were diplomatic in their approach.

In office, though, President Trump proved almost as unnerving to allies in the Middle East as President Obama. His policy was fitful, inconsistent, unpredictable. He flip-flopped on US policy toward Qatar, then he flop-flipped.

One day he seemed to be plunging into outright war upon Iran; the next, he was inviting Iran’s Supreme Leader to tea. Mr Trump waged fierce war against Isis, then betrayed former anti-Isis allies to the Erdogan regime in Turkey. He never said No to Israel, disregarding Arab allies’ support for Palestinians.

That same fitfulness that has alarmed Arab allies is now bringing the Trump presidency to a tumultuous finale. The coronavirus pandemic has cast tens of millions of Americans out of work. Racial protests roil American cities. Mr Trump’s approval rating — seldom much above 45 per cent — is now ebbing into the 30s. The presumptive Democratic nominee, Joe Biden, is running 10 points ahead of Mr Trump in most polls — and running even with him in formerly rock-solid Republican states like Texas. Republicans rightly worry about losing their majority in the US Senate. A new era of Democratic dominance at the federal and state level beckons.

But the Democratic party is likely to return to power in a very different frame of mind than in 2009-2010, the last period in which it held all three of the US’s elected branches of federal government.

Two differences may be of special note to Gulf partners who have invested so heavily in the Trump presidency: the environment and Iran.

In 2009-2010, climate policy was a subordinate matter for Democrats. A decade ago, a climate bill passed the House but faltered in the Senate, never to rise again. The Obama administration later took some executive actions on climate. But always, climate took third or fourth place after other, higher priorities.

This time, climate will be a primary issue for Democrats. Over the past decade, climate concern has surged among Democratic voters. Almost 90 per cent of them regard climate as a matter of highest urgency. Joe Biden tip-toed around the “Green New Deal” endorsed by other Democratic candidates for president. But big changes are coming even so. In fact, they have started already.

US petroleum consumption peaked in 2005. While natural gas consumption has risen, it has done so only to offset the decline in coal-burning. The next decade will be greener though: US renewable output is expected to double to almost 30 per cent of all electrical production by 2030.

Oil and gas will continue to matter for decades to come, of course. Mr Biden’s energy plans project that the US will continue to burn some oil, some gas, and some coal until 2050. China, India and other emerging economies will surely need even longer to transition — although their pace of change may be forcibly accelerated by developed-world tariffs on carbon-emitting goods and services. For a long time to come, the Gulf’s resources will remain of interest to great powers.

Still, as oil and gas become less central to the US economy, the Gulf region could become less central to US foreign policy. Since 1990, the US has been drawn into two major wars and uncountable other military engagements to protect the Gulf.

Mr Trump, supposedly a less interventionist president, twice came to the verge of war with Iran, in July 2018 and December 2019. It is hard to imagine that level of US engagement continuing in the 2020s, no matter how important energy from the Gulf remains to non-US consumers.

For both the Obama and Trump administrations, Iran ranked at the top of US international priorities: for Mr Obama, as a target of transformational diplomacy; for Mr Trump, as a target of coerced regime change.

Both Mr Obama and Mr Trump came to office determined to reverse every action of their immediate predecessor. Mr Obama saw himself as the anti-Bush: if George W Bush linked Iran in an “axis of evil,” Mr Obama would appeal to Iran as America’s new preferred regional partner. Mr Trump, if possible, reacted even more fiercely to Mr Obama than Mr Obama had reacted to Mr Bush, ripping up the nuclear deal signed by Mr Obama even before Mr Trump had any very clear idea what to do instead.

Facing a global pandemic and economic crisis, that level of focus on one mid-sized country in one region is simply not sustainable.

President Trump raised US defence spending to almost $750 billion a year over President Obama’s final budget of $600bn. That was exorbitant enough before coronavirus. In the face of terrifying post-virus deficits and debt, defence spending will have to be reduced — and that implies backing away from as many confrontations as possible, including the confrontation with Iran.

President Obama failed to make good on his promise of a “pivot to Asia.” A President Biden may be equally frustrated by events. But a “pivot from Iran — and from the Gulf generally” — that seems in the cards, driven less by conscious policy choices and more by the overpowering logic of America’s need to get its own house in order to meet the challenge of China’s rise.

As the US depends less on fossil fuels itself, it may become more assertive about pushing other countries to follow, either by diplomacy or by tougher means like higher tariffs on exports from high-polluting countries.

The rulers of Iran will not reform their behaviour just because the attention of the United States is fixed elsewhere. Very likely, a Biden administration will revive a diplomatic approach to curbing the Iranian nuclear programme. But a strategic challenge that ranked near the top-of-mind for Presidents Obama and Trump will be shelved in a lower drawer under a President Biden. The problems of the region are often blamed on American mistakes. A more inward and distracted America may have occasion to make mistakes in the 2020s and 2030s. That may create more opportunities for Iranian mischief — and more invitations for China and India to exert their own power in a region that finds itself looking beyond the American moment.