IN A RECENT BLOG the former Downing Street aide Damian McBride recounted a conversation with his boss Gordon Brown in 2004, just after Tony Blair had announced his intention to serve a full third term. “I’ve already had seven years,” a gloomy Brown told McBride. “Once you’ve had seven years, the public start getting sick of you. You’ve got seven years when you’ve got a chance to get people on board, but after that, you’re on the down slope.”

What McBride’s blog termed a “seven-year hitch” stuck in my mind, not least because it appears to have lots of supporting evidence. As McBride noted, by 1987 even Margaret Thatcher’s personal popularity was on the slide; the same could be argued of Tony Blair by the 2005 general election. And although he hasn’t been Prime Minister for all that time, David Cameron has also been a high-profile Tory leader for approaching seven years.

It also applies, or rather will apply, to Alex Salmond. By the time Scotland goes to the polls to decide its constitutional future in the autumn of 2014 he will have been First Minister for more than seven years. If the Brown dictum is accurate, and assuming Salmond is firmly associated with independence in the eyes of Scottish voters, then the omens aren’t good.

There are already (admittedly anecdotal) signs the public may be growing weary of the once mighty Eck. When he belatedly tried to capitalise on Olympic pride with a victory rally for “Scolympians” in Glasgow he was booed not once but twice. And on a recent trip to the United States Salmond was booed again, although not, one assumes, by people planning to vote “no” in 2014 (because they won’t be able to).

Alex Salmond, let us remember, has been around for a very long time. He first impacted on public consciousness when he famously interrupted Nigel Lawson’s tax-cutting 1988 budget (oh, the irony). In Scotland it established the young MP for Banff and Buchan as an anti-Poll Tax campaigner, while the rest of the UK took note of a spirited and articulate Scottish Nationalist. Salmond even came to the attention of a young George Osborne, who was listening to radio coverage of the Budget on a bus to school.

Salmond has also been leader of his party for a total of 18 years (he was on sabbatical, remember, between 2000-04), a length of time unparalleled by any UK party leader since the Second World War. Having always punched above his weight in media terms, Salmond’s public standing – until 1999 – far exceeded his party’s electoral success. He has been a grinning, witty and shrewd fixture of Scottish politics for more than a quarter of a century.

His approval ratings also remain remarkably high, but then Salmond enjoys a very specific sort of popularity. Asked who best “stands up for Scotland” he wins hands down, but if voters are asked if they agree with his vision for an independent Scotland then it’s two-to-one against. So Scots like Alex Salmond, but they only like him in a particular setting.

That context is the halfway house between full government and opposition otherwise known as devolution. As First Minister Salmond is both in government and in opposition, allowing him to take credit for all the goodies (free benefits) and blame London for everything that goes wrong (cuts). In such a context a politician with opportunistic flair thrives, as does the Conservative Boris Johnson as Mayor of London.

Take both men out of their comfort zone and I suspect they’d find relentless populism a lot harder to pull off, which is why I can neither see Boris as leader of the Conservative Party nor Salmond as Prime Minister of an independent Scotland. At the moment they enjoy the best of both worlds, but take their careers to their logical conclusions and they become just another couple of politicians.

I’ve argued before that the SNP are victims of their own success, but none can feel that more acutely than Salmond, particularly when he adds his signature to the Edinburgh Agreement (or should it be the St Andrew’s House Agreement?) this afternoon. This, arguably, commits him to holding a referendum he neither wanted nor can actually win, for most Scots seem to have made up their minds on independence years ago.

And if “no” votes outnumber “yes” – perhaps to a significant extent – then I really can’t see how Alex Salmond can continue as SNP leader and First Minister. Sure, Nicola Sturgeon is nominally in charge of the referendum, but it is Salmond the electorate equates with independence, so if the ballot fails then it will be associated with him and him only. Gordon Brown’s seven-year hitch will have claimed its next victim.