What if Hillary Clinton can’t run? It’s a question that must be asked, even if the New York Times and much of the American mainstream media has been unwilling to ask it. Until now, that is. Clinton’s collapse – or ‘medical episode’ – during a 9/11 memorial service has brought the issue of her health to everybody's attention.

Americans will be asking themselves how, if she can’t make it through a memorial service, she will cope with the rigours of four years as Commander-in-chief. Moreover, the conspiracy of silence surrounding her health troubles does add to the general idea that the media and the Washington elite are willing to cover up, to mislead the public in order to win an election. This will feed Trumpmania and excite the growing part of the electorate who feel the whole damn system is rigged.

The whispers about her ill-health have long amounted to more than just malicious right-wing gossip. Clinton turns 68 next month and at times during the campaign she has seemed exhausted and unwell. Today's on-camera collapse looks serious -her doctor afterwards put it down to pneumonia. As even the New York Times says, this is not a trivial diagnosis for anyone over 65 as older sufferers "can experience confusion" as well as coughing, fever and low energy and "it can be weeks before a middle age person fully regains strength". Ever since her fall and blood clot in her head two years ago, Hillary has shown signs of not being quite right.

With the immense physical and mental stress and strain that comes with a presidential campaign, it's perhaps not surprising that a 68-year-old is struggling to cope. Trump is 70, of course; his doctor said he would be the "healthiest individual ever elected to the presidency" which is obviously untrue. But there are few legitimate question marks surrounding his physical capacities. Hillary's case is different. Her coughing fits (she had another a few days ago) are becoming a trademark of her campaign.

What if Clinton now has to accept that she is not physically up to the demands of presidency? If she stands down, then what? We are in uncharted and very choppy waters. The party would have to choose another candidate and the nomination would most likely go to Tim Kaine, her 58-year-old running mate, who then would quite quickly become favourite to win the presidency.

Kaine doesn't exactly thrill. He is a fairly middle-of-the-road machine politician: liberal but not progressive in the Bernie Sanders-mould; a traditional Democrat, not a more conservative 'blue-dog' candidate such as Jim Webb. He even admits to being boring in interviews. But he can be likeable -- unlike, say, Hillary Clinton or Donald Trump -- and likeability tends to win big elections. He could even be the only person now who can stop Donald Trump reaching the White House.

Hillary's withdrawal would in fact be terrible news for the Trump campaign. Trump’s greatest advantage as a presidential candidate is not his willingness to say the hitherto unsayable on immigration. It’s that he is not Hillary Clinton, who is widely and massively reviled.

But if Hillary Clinton suddenly has to withdraw from the race – and after today that is not too remote a possibility – then Trump’s trump card is gone. It's still mere speculation, but perhaps we should start considering the possibility of President Tim Kaine.