THE ugly ducklings of the polling world are the “don’t knows” — often overlooked despite their huge potential to change the course of tight elections.

The more people who can’t make up their minds how to vote, the more likely it is that even subtle shifts in public opinion will alter the final result.

In the final days of last year’s general election, Labour and the Tories were neck and neck in the polls, but many people couldn’t decide who to vote for, or even whether to vote.

At the last minute the so-called shy Tories broke for David Cameron, especially in marginal constituencies targeted by Lynton “dead cat” Crosby. If Ed Miliband hadn’t stumbled and instead inspired the undecided, things could’ve gone the other way.

YouGov’s recent survey of Labour members showed only 8 per cent unsure whether to stay in or get out of Europe. But its latest poll of the general public puts 25 per cent on the fence, and the true number’s probably higher because of the shy don’t-knows — no-one likes to look indecisive.

Which means that, while Labour’s position is pretty fixed, there’s everything to play for among the wider population.

Similarly the Tories seem to have made up their minds. YouGov surveyed party members before and after Cameron called the referendum and found the number of don’t-knows holding steady at 11 per cent.

Perhaps it’s not surprising that people political enough to join a party have strong opinions. Tories in particular are known for their black-and-white world-view.

They’re certainly clear who they want as their next leader. With the support of 43 per cent of party members, Boris Johnson is twice as popular as his closest rival George Osborne on 22 per cent (don’t know/won’t votes: 8 per cent).

And by coming Out, the London mayor looks set to secure the backing of enough Eurosceptic Tory MPs to end up on the shortlist of two who will face the members. So he’s now almost certain to be their next leader — the only question is when.

If Britain votes to leave the EU on June 23, Cameron is bound to resign, paving the way for a Boris premiership by party conference in September.

If we vote to stay, but only just, lame-duck Cameron will try to limp on till 2020. But depending on how toxic the referendum turns, he could quickly face a challenge — 50 Tory MPs is all it takes to trigger a contest.

And even if the Leavers lose badly, Johnson might still have enough support to make the ballot by the time Cameron finally steps down. Of course by then, members may have changed their minds, but unless Johnson blows it he’ll crush George among the true-blues.

That’s not great news for Labour — the London mayor’s popularity with the public is at Olympic levels compared to the man who won a gold medal in being booed.

Will Jeremy Corbyn find him a more formidable opponent than Osborne? Certainly, Jez’s sheer decency is the perfect counterpoint to the Chancellor’s creepiness. In a battle between Jesus and Dracula, the smart money’s surely on the saviour.

But Johnson is a different beast altogether. He’s humorous and charismatic, and manages to hide his elitist, far-right, born-to-rule mindset much better than his rivals.

Like Donald Trump, he sells himself as a rebellious and outspoken man of the people. But his real agenda is to celebrate inequality, or as he explains: “The harder you shake the pack, the easier it will be for some cornflakes to reach the top.”

People don’t like what they see when his mask slips, so he is clearly beatable — but it all comes down to the don’t-knows, even if they’re currently less likely to vote. Who are these people holding the fate of our nation in their hands?

That’s who the Labour Party should be polling right now. What do they like about Johnson and what do they hate? What would make them turn out to vote and who would they vote for if they did?

Johnson may have the “Heineken factor” — he can reach parts of the electorate other politicians can’t — but his “Marmite effect” means voters either love him or hate him.

And, like Trump, that’s his biggest weakness. As many Londoners have discovered, it’s easy to reach “peak Boris” — the point after which his roguish charm turns off more people than it attracts.

Labour’s best bet is to ride this anti-Johnson backlash, and with Sadiq Khan in City Hall it would be easy to expose his shoddy mayoral legacy.

Corbyn will also find beating Johnson lighter work with the help of his left-hand man, the “people’s Chancellor” John McDonnell. While top Tories tear into each other over Europe, the Jez-and-John double act continues to impress.

Like the bad cop to Corbyn’s good, McDonnell can be ruthless enough to hit an increasingly isolated Johnson where it hurts. He knows how to be a bit nasty when absolutely necessary, in the nicest possible way, and what’s required to clear Crosby’s dead cats off the table.

And when it comes to bringing down BoJo the clown, it can’t hurt to have half of the dynamic duo happy to “wear a proper suit, do up his tie and sing the national anthem.” McDonnell plays his part to perfection, and the don’t-knows probably appreciate that.

• Chat to Charley on Twitter: @charleyallan.