Why would Jeremy Corbyn be unable to lead Labour to a general election victory? One of the biggest problems that Corbyn’s opponents face is their inability to give a persuasive answer to this question.

Alastair Campbell says in The Independent today that “a country that decided to reject Gordon Brown, Neil Kinnock, Ed Miliband and Michael Foot is unlikely to elect someone to the left of all of them”, but he didn’t say much about why not.

So let me try.

I find Corbyn’s foreign policy positions hard to accept. He is anti-American, anti-Israel and soft on Hamas and Hezbollah. He has not given good answers or any answers at all to questions about his associations with people who have been called anti-Semitic. But I accept that these mean nothing to a lot of people, and that many of the general electorate are pretty anti-American and critical of Israel in any case.

His views on Ireland and the IRA are harder for him to gloss over, but the argument that Tony Blair negotiated with Sinn Fein, the IRA’s political wing, however dishonest, gives him cover. Anyway that is mostly in the past now.

Then there is scrapping Trident. That is not quite such an obvious vote-loser as it was in the 1980s, when Labour’s policy was of one-sided nuclear disarmament. An argument can be made that Trident is a relic of the cold war, and is no longer needed. I don’t agree with it – I want to keep nukes in an uncertain world and their annual cost is not great – but many reasonable people do. Not that many, however. Forced to choose between keeping nuclear weapons and getting rid of them altogether, 56 per cent said keep them and only 29 per cent said give them up in 2013.

Opinion polls seem to suggest that renationalising the railways and energy companies would be popular, but I think those findings – unlike those on Trident – are superficial. Support for renationalisation is mostly a way of saying trains and energy bills are too expensive. Profit is still a dirty word in this country, but when it comes to it, most people accept that spending billions on public ownership is the wrong priority.

Corbyn’s promise to reopen some coal mines is mostly treated as a joke. Whatever one’s opinion about the miners’ strike of 1984-85, no one wants to send young people down the mines to do that dangerous work.

His hostility to academy schools will be unpopular with the parents of children who go to them. If they are good schools with comprehensive entry, he should support them.

His promise to abolish tuition fees is something that Gordon Brown and Ed Miliband both wanted to do but had to admit they could not. It would be expensive, and the cost would be borne by today’s taxpayers, most of whom did not benefit from a university education.

But the big problem with Corbyn’s policy is his approach to taxation, public spending and borrowing. He thinks that in 2020, when the government books will probably be in surplus, the government should tax, spend and borrow more on a vast scale.

As Yvette Cooper says in her speech today, Labour’s economic policy “has to be credible, and Jeremy’s isn’t”. She calls his plan to print money (“people’s quantitative easing”) “really bad economics”, saying “no good Keynesian would ever call for it”.