Referendums are not really very much like buses at all, since they haven't got wheels, 97-horsepower diesel engines or, most of the time, a man saying: "Hold very tight please, ting-ting."
Still, you wait ages for one then two come along at the same time - or at least that seems a possibility. As David Cameron visits Edinburgh today to sign the agreement on arrangements for the referendum on independence, Michael Gove, the education secretary and one of the Prime Minister's closest allies, has voiced the opinion that there should be another referendum. This one would be on Britain's relationship with the EU and, as things stand, he says he'd be inclined to vote to leave.
Another possible point of similarity is that they are both Latin words about carrying everyone; omnibus literally means "for all", and referendum "something needing carried back" (to the entire electorate). That's why it's referendums, by the way, for more than one ballot on single issues; referenda would be lots of things being referred.
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But that is, of course, one of the points about the first referendum under discussion – the one that the Prime Minister and First Minister will announce later today. If the advance reports are correct, Alex Salmond has obtained the right to pick the timing, the wording and the make-up of the electorate for the vote (probably including 16 and 17-year-olds), while Mr Cameron has got his way on the point that there shall be one question. So only one issue – independence or the status quo – is under consideration on the ballot.
This is where there is a comparison to be drawn with Mr Gove's case for a referendum on the EU. His argument is that Britain won't get much out of attempts to renegotiate our relationship with Europe unless there is a credible threat to leave altogether, and that there is nothing particularly scary about the prospect of doing that anyway. This strikes me as having the merit of being perfectly true, but the same case could also be made by advocates of devo-plus or devo-max – the policies which, according to the polls, attract the largest body of support from Scottish voters.
The interesting thing here is not the importance of these two referendums for, on the one hand, convinced Scottish Nationalists or, on the other, convinced eurosceptics (whether supporters of UKIP, the Tory Party or, indeed, many traditional Labour voters). It is that they offer a strategic position for those who would not go quite so far, but would like substantial alteration to the status quo.
Until quite recently, one could have been confident that this group would have been the majority on both these separate issues, and indeed, most polling suggests that is still the case. But what is fascinating at the moment is that it is no longer certain, as it would have been even 10 years ago. The rise in support for the SNP and the crisis in the eurozone, which has so completely vindicated the eurosceptics' position, make the outcome of both polls impossible to predict with total confidence.
And that, of course, is in itself the principal thing that makes them viable as mechanisms for achieving change other than the change which is ostensibly on offer. It is the fact that Scots might just conceivably vote for full separation from the rest of the United Kingdom that has prompted the Government to offer additional powers, if only they choose to stay.
And it is the prospect that Britain might decide to emulate Norway and Switzerland, which are managing perfectly well outwith the EU, that might be enough to wrestle back some of the powers that Brussels has appropriated over the years and which, for all their bluster, no British politician has succeeded in retrieving.
If we assume that most Scots would like more powers for Holyrood, but not independence, and most Britons more sovereignty, but not to leave the EU altogether (both, as I say, no longer assumptions which can be made with complete confidence), we are, however, left with a predicament. Their best chance of success comes when they not only significantly outnumber those who support the status quo, but when at the same time those who advocate even greater change – the Nats and those wanting to leave the EU completely – garner enough support to make their objectives real possibilities.
So if you want devo-max, say, or to withdraw from EU directives while remaining in a trade alliance, it is in your interests for the SNP or UKIP to do well, so well that their winning is entirely credible – so long as they fail when it comes to the point. And whether this balancing act works depends on your having the nerve to deal with the consequences of miscalculating. Mr Gove's assessment that you need to threaten to leave the EU to get real repatriation of powers depends on his other judgment – which is that leaving the EU wouldn't be all that terrible anyway, and you'd at any rate be better off than you are with things as they currently stand.
The campaign which kicks off today is about that calculation for another union, now that the procedural niceties have apparently been sorted out. Over the next two years, the arguments for the status quo and for independence will sway a few, while those who already hold strong views are unlikely to shift them. But the real focus of the campaign is on people who plan to vote against independence, but only if they feel that real additional powers will be forthcoming.
The changes in the Scotland Act and the assurances of all the Unionist parties are designed to secure that group, and the polls indicate a comfortable lead just now. I would guess that at the moment many people might take a chance in an EU referendum, but not the independence one.
At a pinch, they wouldn't mind being separated from the French and Germans because they think that they are foreigners anyway, but they're not quite so keen to take a chance on the same happening to the English, Welsh and Northern Irish. But a lot can happen in two years. Hold very tight, please.