At 81 years old Joe Biden has a wealth of experience to draw on. There is just four years difference between him and Donald Trump. And his rival is as well known as the president for misspeaking and making gaffes. Yet something has changed.

Unease has been growing about Biden’s perceived frailty and his mental acuity – and that was before a bombshell report by the US Justice Department’s special counsel. In the report Robert Hur, the Republican special counsel, said Biden would not face criminal charges for mishandling classified documents. The ruling should have been good news for Biden, except the reason given – that Biden would appear to any jury as a “sympathetic well-meaning elderly man with a poor memory” – was so damning. Biden hit back in a press conference slamming the inference that he was old and doddery. But as he did so he managed to mix up the names of the presidents of Mexico and Egypt.

Biden’s supporters argue that Trump is just as prone to making mistakes and is hardly more reliable, the Guardian’s Washington DC bureau chief, David Smith, notes. Yet somehow the mistakes Biden makes are all taken to be a sign he is losing his grip. Michael Safi asks why the same charges against Trump don’t stick and how Biden’s campaign can prove the president is fit and sharp enough for another four year term.