In his speech on Scottish independence, David Cameron declared: "The outcome is still up in the air." So are the questions provoked by his intervention. Why make his first major address on the issue from a venue in London? I can see why it was considered a bright idea to go to the Olympic Park when he wanted to invoke the Games as a reason to celebrate "Britishness" and advance the "emotional case" for keeping the United Kingdom together.

But Number 10 might have anticipated that Alex Salmond would jibe that the prime minister was "a big feartie" for sharing his thoughts about the destiny of Scotland from a distance of several hundred miles away. The Nationalist leader was given another opportunity to press for a head-to-head debate between the two men, a demand that he makes because he thinks the chances of persuading Scots to vote for independence are enhanced if he can turn it into an argument between him and a Tory prime minister and a demand that Mr Cameron resists because he grasps full well that this is Mr Salmond's game.

In fairness to the prime minister, I am sure Mr Salmond, a resourceful man when it comes to abusing opponents, would have found a way to scorn him wherever Mr Cameron chose to speak. It is to the Tory leader's credit that he is more aware than many of his party that there is scant appetite in Scotland for listening to lectures about their future from the English, especially English Tories and even more so southern English Tories with a private school accent. He acknowledged as much in the speech. As a leading Scottish Labour MP puts it: "Cameron is acutely conscious that him parading around Scotland will very likely turn people away because he's seen as the quintessential Tory toff."

Why deliver the speech now? Up until this point, and largely for the reasons outlined above, the prime minister has been hesitant about engaging with the debate that rages north of the border, even though it is inescapably obvious – about this Mr Cameron is quite right – that the departure of the four million citizens of Scotland will have implications for the 59 million people who live in the rest of the United Kingdom. It has been suggested that English politicians haven't got stuck in because they are complacently assuming that, when Alex Salmond's optimistic visions of an independent Scotland collide with making an irreversible choice in the polling booth, the Scots will prove themselves risk-averse and decide to stick with the union they know. There used to be some truth to this. The Westminster political classes were largely lazily taking it for granted that the union was safe. But not any more. Certainly not in the case of Mr Cameron. When visitors to his study at Number 10 ask him how he expects the referendum to turn out, he reaches out and touches wood, not a gesture that suggests he is entirely confident about the outcome.

Since the Scottish government published its white paper on independence, opinion polls have suggested growing support for secession. Not enough yet to produce a majority in favour of independence, but sufficient to cause increasing anxiety among pro-union politicians.

"It is going to be close," predicts a senior figure in the Better Together campaign. Even if independence is defeated, there is mounting concern that it will only go down by a narrow margin, an outcome that could lead to years of instability. "What I fear is a 55-45 result," says one senior Tory. A resounding rejection of independence, as many had thought would be the case at the outset of the campaign, might have settled the issue and secured Scotland inside the UK for a generation. A close result would give the Nationalists every incentive to keep returning to the subject. There could be the prospect of foreverendums. While the unionists have to win every time to preserve the UK, the Nationalists only have to win once to secure independence.

It is quite wrong to say that politicians in the rest of the UK are indifferent to the choice that Scotland will make. Labour is very exercised, not least because its senior ranks are heavily populated with Scots and it relies on Scotland to deliver it a large slice of Westminster MPs. The Tories might be expected to be much more ambivalent about a country that has shown such a massive disinclination to vote for them since Margaret Thatcher and her policies turned so many Scots away from the Conservatives. Scotland has some of the richest postcodes in the United Kingdom, but they just won't vote Tory. At the last UK general election, the Scots elected precisely one Tory MP, which means there are more Chinese pandas in Scotland than there are Conservative MPs. In terms of sheer electoral self-interest and crude power politics, the Tories are arguably better served if Scotland votes to go its own way this September. Rerun the result of the last general election without Scotland and absent the 59 MPs it sends to Westminster, then in the reduced House of Commons of 591 MPs, the Conservatives would have 305 seats and a majority of 19. Not marvellous, but a great deal better, or so Tories might think, than lacking any majority at all and having to be in coalition with the Lib Dems.

It cannot be said that Mr Cameron is motivated by what might best serve his party's raw electoral interests. He sounded entirely genuine in his passion when he declared that he "could not bear" to see the United Kingdom "torn apart". He referred to his own Scottish ancestry, giving voice to the ties of blood and history that he feels. He also senses very powerfully that his own historical reputation is at stake. He leads what used to be known as the Conservative and Unionist Party. Some of the blue tribe don't much care whether or not Scotland departs. Some would positively welcome a divorce. But not Mr Cameron. In future editions of Our Island Story, the patriotic tract that he referenced in his speech, he does not want to go down as the prime minister who "lost Scotland".

His speech was also an implied recognition that the case for the union has been made in a tone that is too negative, especially when the speakers have been his Tory colleagues. These have included William Hague trying to scare Scots with the notion that they will find it harder to sell their whisky if they leave the United Kingdom; Philip Hammond attempting to shiver their wallets with the threat that they will lose defence contracts; George Osborne suggesting that an independent Scotland will be too financially feeble to support its banks. The governor of the Bank of England, Mark Carney, went north recently to drizzle some cold water over the feasibility of an independent Scotland remaining in a currency union with England, Wales and Northern Ireland by retaining the use of sterling. Even when they have a point and even when it is expressed with finesse – the governor did have a case and he did deliver it with subtlety – it has come over as an essentially miserabilist and technical argument.

Whether and how much to play on fear has from the beginning been a cause of tension and dispute within the pro-union camp. One senior Labour figure argues: "It is still a strong card, people's doubt and anxiety about what independence would mean in practice. The fear factor does matter." Private polling conducted for the Better Together campaign suggests that there is one thing that the Nationalists could do to shift a substantial number of the undecideds over into the pro-independence column. That would be to address the anxieties of the undecided about what an independent Scotland would really look like by promising a second, confirmatory referendum after separation negotiations are complete. Mr Salmond may have missed a trick by not offering a second referendum.

For others in the pro-union camp, fear-mongering is ineffective and even counter-productive because it just raises the hackles of Scots when it implies that they don't have it in them to be an independent nation. Wisely, Mr Cameron did not make that claim. He instead appealed to the English, Welsh and Northern Irish to phone, text or email their Scottish friends to tell them: "We want you to stay." The implication here is that the Scots simply need to be told how much they are loved. Some will find that risible.

But there are Labour campaigners on the Better Together side who think the prime minister is on to something. They point to Quebec where it is contended that a pivotal role was played in preventing the province from breaking away by the Canada United campaign, which marshalled voices from the rest of the country to tell the Quebecois how important they were. One senior Scottish Labour MP argues it will be ultimately helpful for the cause of the union if the rest of the UK engages with the debate. "If there is a general feeling in Scotland that the view in England is, 'Sod off then', there's a thrawn side of the Scottish character that will respond, 'OK, we will sod off then'."

David Cameron is betting that, while Scots may not care for him personally, they do still care what England thinks.