Tonight, Donald Trump and Joe Biden will face off in the first of two debates before either of them has even been officially nominated by his party. The timing is strange — these events have typically been held later in the year, after the hoopla of the party conventions and when more voters are paying attention. But Trump boasted that he would debate Biden “anytime, anyplace,” and the White House incumbent took advantage of that by proposing a friendly venue (CNN) and the earliest possible time.

Both campaigns also saw fit to abandon the Commission on Presidential Debates, which has organised these events since 1988. Republicans had long been annoyed at the Commission’s perceived bias, dating back to CNN anchor Candy Crowley’s impromptu (and incorrect) fact-check of Mitt Romney in 2012. Democrats were angered that Fox News’s Chris Wallace didn’t fact-check Trump enough in 2020. So they’re free-styling this time, and CNN has jumped in to fill the void.

Often, the presidential debate is the first prolonged, mostly unscripted introduction that many Americans have to the candidates. That will not be the case here. Biden and Trump have each served a term as president, making them better-known than the typical nominee, and each has been campaigning for some time now.

Oddly, it is the sitting President who has the most to gain from this debate. Almost since his presidency began, Biden has been plagued by rumours that he is too old and enfeebled for the job. This event will put those rumours to the test. The President will therefore need to look like he is in control of the situation and in command of the facts and arguments needed to persuade the voting public.

Meanwhile, Trump could win just by presenting himself as calm and collected — but this has always been true and always easier said than done. In their first meeting in 2020, Trump interrupted and spoke over Biden so much that the debate became difficult to watch. CNN’s Jake Tapper (the moderator of the upcoming debate) called it “a hot mess inside a dumpster fire inside a train wreck,” and “the worst debate I have ever seen”. If Biden’s task is to keep it together, Trump’s is to not get in his own way.

Unless some truly unusual incident occurs, though, the debate itself might not change much. In years past, candidates have seen a post-debate “bump” in the polls for a good performance. Romney briefly surged into the lead in 2012 after his second debate with President Barack Obama, before regressing gradually back to where he began.

The early timing of this debate makes it even more likely that any short-term effect will fade, and the nature of the 2024 race suggests that even a bad performance by one or the other will be shrugged off in time. These candidates are better-defined in voters’ minds than pretty much any in modern history. Everyone of voting age has lived in a country governed by Trump and then by Biden. They have taken the measure of both, and most are not persuadable. Even Trump’s 34 felony convictions barely moved the polling needle.

The most persuadable voters may be the “double haters” — those who view both men unfavourably. Many of these will never vote for one of the candidates and will probably not vote for the other. Here, perhaps, is what remains of the swing voters: waiting to be convinced that one of these two candidates is slightly less worthy of their contempt.