Talk of a by-election in Portsmouth South has been growing, fuelled by allegations against MP Mike Hancock. And, in a speech to the parliamentary press gallery lunch yesterday, Nigel Farage claimed Ukip could win it. The reasoning is simple: Ukip are on the up, and they came within 2,000 votes and 5 percentage points of a win in Eastleigh, so surely they can go over the top in another Hampshire by-election where the Lib Dem incumbent has had to step down amidst a scandal.

Of course Ukip could win — but its chances may not be as high as that reasoning suggests. Indeed, Farage himself seems to think his party would be more likely than not to lose. He said ‘I would think that the odds on Ukip winning it would be 2/1, 3/1 at Ladbrokes and elsewhere.’ That prediction raises two questions: would the bookies offer those odds? And should you take them if they do?

The Eastleigh by-election provides a reasonable point of reference. When Chris Huhne resigned, Ladbrokes initially offered 6/1 against a Ukip victory, but immediately lengthened those odds to 12/1 when Farage declared that he wouldn’t stand. They went further out to 25/1 before tightening during the closing days back to 6/1 on polling day. Are there good reasons to think Ukip would be that much more likely to win in Portsmouth South than they were in Eastleigh? (Odds of 6/1 translate to a 14.3 per cent chance; 3/1 to 25 per cent and 2/1 to 33.3 per cent.)

Ukip do seem to have moved up slightly in the national polls since the Eastleigh by-election: the Polling Observatory aggregation put together by Drs Rob Ford, Will Jennings and Mark Pickup has them rising from 9.3 per cent on 1 March to 11.2 per cent on 1 April.

But even if the national picture is better for them when the by-election takes place, the demographics of Portsmouth South look less favourable to Ukip than Eastleigh’s. In particular, Portsmouth South is much younger than Eastleigh: the mean age is 35 compared to 39.5. In Eastleigh, over-60s make up 27.9 per cent of the adult population; in Portsmouth South they’re just 19.6 per cent. Partly because it contains the University of Portsmouth, Portsmouth South has 6,900 full-time students to Eastleigh’s 2,200. 38 per cent of the adult population is under 30, compared to 18.4 per cent in Eastleigh.

This matters because Ukip relies on older voters to greater extent than the other parties. In February, YouGov found that 48 per cent of Ukip supporters were over 60, while just 8 per cent were under 30. Of course, they’re helped by the fact that older people are much more likely to vote — especially in low-turnout by-elections. But we can expect the average Portsmouth South voter to be at least slightly younger — and therefore less likely to vote Ukip — than the average Eastleigh voter.

And, like in Eastleigh, the Liberal Democrats dominate at a local government level — they hold 17 of the 18 City Council seats in the constituency (the Tories have one).

So, rather than ‘Ukip nearly won Eastleigh, surely they’ll win Portsmouth South’, a more realistic framing might be ‘Ukip didn’t win Eastleigh, surely they won’t win Portsmouth South’. A Ukip victory is certainly possible, but I’d need much more generous odds than 2/1 or 3/1 to bet on it.

Tags: By-election, Eastleigh by-election 2013, Liberal Democrats, Mike Hancock, Nigel Farage, Portsmouth South, UKIP