Where is Joe Biden’s presidency most likely to hit the rocks?

His management of the pandemic will (hopefully) be a distant memory the next time America votes. His economic aid package, for all its shortcomings, is popular. While economic circumstances may be dire now, the president will probably be overseeing robust growth fairly soon, as a vaccinated US comes back to life. There is a risk of overheating, but that is a nice problem to have.

On healthcare, Biden’s plan to “protect and build on” Obamacare is unlikely to ruffle feathers. There’s foreign policy, of course. Many an American president has found himself bogged down by diplomatic and military trouble, and China and Iran pose serious headaches for the administration.

But if we are asking what will do for Biden at the polls, then the cliché that Americans don’t vote on foreign policy probably holds true. With the admittedly broad caveat that events could scramble these political calculations overnight, the trickiest area for the new president will likely prove to be immigration.

The big immigration questions — Who gets to come to America? What should be done about those living here illegally? How should the immigration rules be enforced at the border? — were fraught before the Trump era. Thanks to a knotty mess of economic, social, humanitarian and political considerations, they remain as unresolved today as they were a decade ago. And the last four years have only made constructive progress more difficult. Immigration has been further toxified and is too often framed as a black-and-white clash between “build the wall” and “open borders”.

Some parts of Biden’s immigration agenda aren’t especially complicated or politically tricky. The low-hanging fruit includes this week’s reversal of a freeze on issuing new green cards imposed because of the pandemic, a straight-forward and sensible move. Earlier this month the president established a task force to help reunite 600 children with their parents after families were broken up at the US-Mexico border, another no-brainer. The travel ban on a group of Muslim-majority countries has been lifted.

However, the president has already gone further than these post-Trump correctives, introducing a comprehensive immigration bill on the first day of his presidency. The US Citizenship Act would, among other things, offer a pathway to legal status for the estimated 11 million undocumented immigrants living in America, increase skilled immigration, change the the word “alien” to “non-citizen” in US law and increase the number of green cards awarded by lottery every year.

Progressives were thrilled by the proposals, which are all but certain to sink in the 50-50 Senate. The boldness suggests a cynical calculation: that compromise is impossible. Better to keep the party happy with a comprehensive package that never becomes law than have a serious go at reforming the system.

But whilst legislative plans can be shelved, Biden cannot escape the on-the-ground dilemmas of enforcement. This week, the government reopened a Trump-era migrant child facility in Carrizo Springs, Texas, to house unaccompanied teenagers crossing the border from Mexico, prompting criticism from the left and cries of hypocrisy from the right.

These are early signs that Biden is paying for the way in which Democrats have presented immigration as an oversimplified morality play for the last four years. Like it or not, the president can’t escape the relationship between changes in enforcement policies and who attempts to enter the country. For example, the new administration has decided to exempt minors from a Trump-era expulsion policy for anyone crossing the border. And so, sure enough, more unaccompanied minors have attempted to cross the border.

A booming post-Covid economy, relaxed enforcement of the existing rules and talk of amnesties and broader liberalisation is a recipe for a surge in arrivals at the southern border. That will force Biden into a choice: turn a blind eye to unauthorised migration or toughen up enforcement. Neither will be especially palatable.

The context that makes this so difficult for Biden is a Democratic Party whose approach to immigration has hardened at an astonishing pace. For years, Bernie Sanders, the granddaddy of American progressives, opposed high level of immigration because of he thought it hurt the wages of US workers. Open borders, he argued only half a decade ago, were a Koch brothers-funded plot to further tilt the balance against the American worker. These days, he is mum on the subject. In 2006, Barack Obama wrote in The Audacity of Hope: “When I see Mexican flags waved at pro-immigration demonstrations, I sometimes feel a flush of patriotic resentment. When I’m forced to use a translator to communicate with the guy fixing my car, I feel a certain frustration.” Today, such an admission would mean instant cancellation in mainstream Democratic circles.

Part of this change is thanks to the way the Trump years turned immigration first and foremost into a moral issue. But it is also informed by an electoral calculation: the more Democrats see their future as a party built on Hispanic votes, the more they are willing to become the out-and-out pro-immigration party. But last year’s election, in which Trump did surprisingly well among non-white voters, should cast doubt on the assumptions American liberals make about minority voters. This lesson was clearest in the overwhelmingly Hispanic Texas border towns that I reported from for our December issue. Those South Texas counties saw some of the biggest swings towards the Republicans anywhere in the country.

If a border crisis does materialise, it is far from clear there is a way for Biden to keep everyone happy. He will either have to take action that many on his own side see as unconscionable, or allow an out-of-control situation to persist, undermining support for legal migration, failing in the basic task of maintaining the integrity of the country’s borders and sowing the seeds for the next Republican victory.