David Cameron may have injected some passion into the pro-UK campaign with his speech at London's Olympic velodrome, but once again he has opened himself up to calls for a TV debate with Alex Salmond.

The First Minister was quick to point out that while the Prime Minister said he would "fight with all he has" to keep Scotland in the UK, that apparently fell short of having a head-to-head debate.

Should Mr Cameron submit? There are arguments on both sides. As Prime Minister, Mr Cameron's writ extends here, even if his popularity does not, and so if he and his Government oppose Scottish independence, he should be prepared to front up to that view by debating with the First Minister, in Scotland. Speeches like yesterday's show he is more than willing to intervene, so why stop short of a TV debate?

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Yet at the same time, the Prime Minister is denied a vote in the referendum, which is only for Scots. With other high-profile TV debates, such as during a general election, only party leaders who are able to cast votes themselves take part. Looked at that way, Alistair Darling is the person Mr Salmond should debate with. Mr Cameron's speech yesterday was also more than a salvo in the Scottish debate, it was aimed at English, Welsh and Northern Irish voters as well as Scots. The Prime Minister obviously has the right to discuss such a major potential constitutional change with a UK-wide audience.

So the arguments go round and round, but what is clear is that calls for Mr Cameron to debate with Alex Salmond will get louder if the Prime Minister makes more such speeches, as he intends to do.

That said, no one who follows the referendum debate even half-heartedly will be under any illusions about why the SNP are so keen to stage a TV clash with Mr Cameron. They know that having an Eton-educated Englishman on screen discussing Scotland's future would produce a powerful emotional reaction among some viewers. "Who are you to come up here and lecture us, posh boy?" would be the general gist. The effect for the pro-UK campaign could be dire. For the SNP, their difficulties over a shared currency and fast-track EU membership would seem much less worrisome after a debate like that. The Prime Minister is the SNP's greatest weapon, and he knows it, which is why he will resist a TV debate to the end.

Meanwhile, Mr Cameron came in for furious criticism from the Deputy First Minister for deliberately invoking the London Olympics in the pro-UK cause. If the Scottish Government used Glasgow 2014 in that way, said Ms Sturgeon, they would be pounced upon for it. That is absolutely true, though the SNP also has form when it comes to using sport for political purposes. There is the small matter of the timing of the referendum, a month after all the Saltire-waving of the Commonwealth Games, and then there was that incident at Wimbledon last summer, involving Alex Salmond and a Saltire.

You are all as bad as each other, voters could be forgiven for saying - and to think there are still seven months to go.