The first - and potentially only - debate between the 45th and 46th presidents of the United States constituted a clear win for Donald Trump, as far more viewers focused on Joe Biden's apparent physical and mental infirmities than Donald Trump's evasions and trafficking in partial or outright lies. The question now, though, is what, if any, difference it will make in the presidential election that is now just four months away.

The debate most likely increases the odds that Trump will occupy the Oval Office come noon on January 20, 2025. Going into Thursday night's debate, Trump was slightly ahead in many of the national polls and, more importantly, in the half-dozen "swing" states that will most likely determine the election's outcome. The debate only added to this advantage.

The context favors Trump. This year has already proven to be a tough one for incumbents seeking re-election, as outcomes in India and France have demonstrated (with Britain up next). Polls also show a low approval rating for the prime ministers of Japan and Canada, which could lead to a change of leadership in those countries. Biden and the US are poised to be consistent with this trend.

Like many of his fellow incumbents, Biden has struggled to manage rising immigration and economic challenges. His failure to deal effectively with the southern border has allowed some ten million men, women, and children to enter the US illegally. Then there are the effects of inflation, something voters are reminded of every time they go to the grocery store or fill their car with gasoline. Biden can point to domestic and foreign-policy accomplishments, but they are less salient to many Americans.

Most critical is the question of his age. Doubts are widely and deeply held that Biden is simply too old for what is arguably the world's most demanding and important job. He is 81, turns 82 in November, and, if re-elected, would turn 86 while still in the White House. And he is an old 81. As the debate demonstrated, he walks stiffly, loses his train of thought, and has a weak and raspy voice. Trump is only three years younger and makes little sense when he speaks, often taking bizarre rhetorical detours, but manages to project a more vigorous image.

Given Biden's better outing at a campaign stop the day after the debate, some believe he can bounce back. After all, Ronald Reagan and Barack Obama recovered from poor debate performances. But that was because they were seen as great orators who simply had a bad night. Biden's problem is that his poor showing reinforced an already-entrenched narrative that will be difficult, if not impossible, to alter. His performance could even threaten to turn him into something of a lame duck, further weakening his influence at home and abroad.

All this said, Biden will be the Democratic Party candidate chosen at the party's convention this August unless he takes himself out of the running and frees his pledged delegates to vote for someone else. Who this "someone" would be - Vice President Kamala Harris, a sitting governor or senator, a member of his cabinet - is anybody's guess.

It is obvious that Biden and his inner circle are resisting calls (including from sympathetic editors at major news outlets) for him to step aside. Neither he nor his loyal lieutenants, many of whom have been close to him for decades, have given any indication that the president will bow out.

Biden's declining political fortunes could well prove to be a drag on other Democratic candidates come autumn. It is possible that a Trump victory could help bring about a Republican takeover of the Senate at the same time Republicans hold the House of Representatives. Together with a Supreme Court that has increasingly demonstrated sympathy for positions supported by Trump and congressional Republicans, this would bring about the American equivalent of a parliamentary system, with power consolidated in the hands of a party that is better understood as radical rather than conservative.

There would be few checks on power strong enough to mitigate this imbalance; on the contrary, Trump's plans to weaken the independence of the civil service, together with his promise to politicize the Justice Department and regulatory agencies, would concentrate power further. Trump would be free to lower taxes, impose tariffs, further restrict access to abortion, ease already loose controls on gun ownership, enforce immigration law as he sees fit, and increase the enormous debt.

Foreign policy would also be vulnerable to significant change, because the US political system gives broad latitude to the executive. It is quite possible that Trump would reduce or even eliminate US support for Ukraine, hollow out US commitments to NATO, give Israel an even freer hand to prosecute war in Gaza and Lebanon and build settlements, refuse to participate in global efforts to combat climate change, and prioritize bilateral trade issues with China over broader concerns with Beijing's behavior abroad.

Elections have consequences, and this one more than most, given that the differences between the candidates far exceed any similarities. In the wake of a debate that appears to have tipped the scales against Biden, and with no way of knowing if someone else will be the Democratic candidate and how he or she would fare, US friends and allies should prepare themselves for potentially major changes come January.

From Project Syndicate