Even Donald Trump appeared almost dazzled by the magical efficacy of one of the mantras of his 2016 campaign. If a rally got a little boring, and he sensed that people might be thinking of leaving, “I just say, ‘We will build the wall!’ and they go nuts,” he crowed.

His “big, beautiful wall” is no closer to reality than it was then, but he continues to relish its powers. It is clear that it will not be paid for by Mexico, as once promised, and will not do what he says it will to protect the United States. But in his highly performative presidency, Mr Trump values its symbolic importance. Unlike that other mantra – “Lock her up!” – its potency is undiminished by his victory. It allows him to present himself as the eternal insurgent, battling for change against the establishment.

This explains the second-longest government shutdown in the US, as Mr Trump demands a $5.7bn downpayment on a pointless and very expensive project. With 800,000 federal employees facing the prospect of their first missed paycheck, growing public discontent about the impact, and no sign that the Democrats will offer him a route out, Mr Trump may be beginning to regret his televised declaration that he would be proud to shut down the government for border security: 47% of voters blame him for the impasse, and only 33% blame the Democrats.

His first Oval Office address on Tuesday is unlikely to shift this. While it invoked the full authority of the presidency, it fitted ill with his style, which may explain Mr Trump’s reluctance to deliver it. (He also seems unenthusiastic about the border trip planned for Thursday.) Though his words at times echoed the hateful rhetoric of his inaugural speech, even his demand to know “How much more American blood must we shed?” seemed oddly half-hearted in delivery. He paid unconvincing lip service to the humanitarian crisis he created.

There were more distortions and lies. In contrast to the administration’s claims, most drugs are smuggled through legal ports of entry; migration continues to fall; and unauthorised migrants are no more likely to commit criminal acts than others, and may be less so. But to Mr Trump, what matters is not whether words are true but whether they are compelling – and 86% of Republicans back the wall. Some around him believe he is wedded to realising it, or some version of it that he can claim as a victory, because of its talismanic qualities. Others believe that the fight itself is the thing. Either way, there is the alarming prospect that he might invoke a national emergency, allowing him to use Pentagon resources for the project. Legal challenges would be almost guaranteed, but if courts blocked his progress he would still be able to position himself as the doughty fighter for the people.

His rallies rang to renewed chants of “Build that wall!” as the midterms approached last year, and his claims about a migrant “caravan” were given undue credence – but only went so far in holding back the Democratic advance. His base alone cannot win him a second term. It is true that the Democrats’ recapture of the House gives Mr Trump an opposition to run against. But their response to the shutdown is a reminder that the election left them not only stronger in legislative terms, but also emboldened to face down the president – dismissing his “temper tantrum” – and to try to make the political weather. Some Republicans have voiced potential support for Democrats’ plans to reopen parts of government.

Yet however this ends, Mr Trump may feel the wall is as useful as ever. His remarks in 2016 made it clear that it was as much about tactics as his strategy of fostering fear and division. It is to be invoked in times of need. Those have arrived. Mr Trump looks increasingly anxious for a deal in the trade war he started with China. Above all, Robert Mueller’s investigation moves closer and looms larger. On the same day that the president spoke, we learned that Mr Mueller has accused his former campaign chairman, Paul Manafort, of sharing polling data with a Russian man linked to Moscow’s intelligence agencies. Is it any surprise that Mr Trump would rather discuss the border? The wall does not have to be real to be useful: the only question is whether its magic is gradually wearing off.