Donald Trump is playing the underdog again. Not because he thinks the world is rigged against him - the FBI’s intrusion with a barrow-load of new emails related (or not) to Hillary Clinton’s email server has changed his mind about that - but because he recognises he might lose.

Polls appear to show him with the momentum. But “don’t believe it,” he told supporters in Florida on Wednesday. “Pretend we’re slightly behind. You gotta get out. We don’t wanna blow this.”

But this implies, of course, that he thinks he has a realistic chance to win.

Well anything can happen in a race as confounding as this. We are talking about a man campaigning for the White House on the same day that a California woman was vowing to step forward and accuse him of raping her when she was thirteen. Talking, not proving, of course. And in the end the woman, known as Jane Doe, decided against going public because, her lawyers said, of threats made against her. But streuth.

Or as President Obama put it campaigning for Ms Clinton in North Carolina. “It's strange how over time, what is crazy gets normalised.”

But let’s pretend we're are in normal-land. Pollsters usually put major store in momentum, especially in the final week of a campaign. The newest ABC News/Washington Post daily tracking poll has Mr Trump tied with Ms Clinton nationally. This is impressive when you consider that the first of the tracking polls two Sundays ago had Ms Clinton 12 points in the lead.

The same polls says they are also getting identical levels of support from within their own parties. Both are getting 85 percent of voters who identify with their respective parties, including independents who lean toward one party over the other.

Things may be getting more perilous for Ms Clinton in some of the battleground states too. Talk of her expanding into previously red states like Georgia, Texas, Arizona and even Utah has started to fade away. Instead her super-PACs are pouring money into states she shouldn’t really have to worry about like Michigan and Wisconsin.

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This makes sense just as it makes sense for Trump-Pence to be expending resources campaigning in those two states. Because if they can break through Ms Clinton’s firewall in either one of them and then do well enough on election day to capture swing states like Ohio, Florida and North Carolina - in declining degrees of likelihood - then he could still win.

Accept that what happens in all the swing states bears some relation to how the national popular vote is trending, then you will understand why Camp Clinton has the jitters.

According to the fivethirtyeight.com, if a candidate wins the national vote by 3-5 per cent, their rival has no realistic path to achieving the 270 needed in the Electoral College. But as that gap gets narrower, so the other side has a better chance of denying them final victory.

It remains the case that more things have to go right for Mr Trump on election day than for Ms Clinton. His underdog thing is not just an act. But to be sure that doesn’t happen, the Clinton campaign has to find a way to reverse his momentum of recent days or at least halt it.