The most amazing aspect of the smooth rise of the SNP from a fringe protest party to an assured and mature party of government has been its new-found discipline.

Just 30 years ago the party was in acute crisis, mired in a viperous factionalism which prompted observers to predict that it could never develop into a mainstream, electable force in Scottish politics. Indeed some thought that the party would be lucky to survive at all.

The crisis, which lasted four or five years, was deep. The left-leaning 79 Group, which included the young Alex Salmond, as well as the party's most charismatic figure, Margo MacDonald, its leading intellectual, Stephen Maxwell, and its most effective orator, Jim Sillars, became a party within the party. Gordon Wilson, SNP leader at the time, decided to expel Alex Salmond and several of his colleagues.

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Mr Wilson was recently asked at a public meeting if he regretted throwing out Mr Salmond and the others.

Not at all, he replied, insisting it had been necessary for the long-term unity of the party.

History has proved him to be correct. Perhaps it's the memory of that bitter time that ensures that the modern SNP are so disciplined. I've heard various hardened political commentators saying, with admiration but also possibly a slight tinge of regret, that it's utterly extraordinary so little leaks from the inner circle.

There is no hint or whisper of dissent or personal animus among the leadership, despite plenty of people looking intently for such division.

The paradox is that although the SNP are relatively young, no other British political party compares with it when it comes to maturity. Partly this is a result of the building up of the party machine in the 1990s and the early years of this century, work for which Mike Russell and John Swinney can take most credit - and partly it is simply innate political decency. The leading figures in the SNP are responsible politicians, they have respect for each other and they - just about - keep their egos under control. Personal ambition is subsumed for the greater cause.

That is not true of any other significant party in the UK. The two parties which form the UK governing Coalition are both notorious for a lack of discipline. This is especially true of the Tories. For them, Europe is just one of several issues which keeps them divided. And right now there seems to be a growing and deeply unpleasant sexism in the party.

Of course there have been policy differences within the SNP - over Nato membership for example, and there are current squabbles about legal corroboration. But in a way such disagreements prove my point. They do not get in the way of the party's unity and its momentum.

This almost uncanny discipline raises two key questions. Is the prize - Scottish independence - so great that it is holding the party together, despite underlying and unseen tensions? An even more significant, if related, question is: in the event of a Yes vote in September, would this unity rapidly dissolve?

For what is happening now is relatively easy. Persuading the Scottish people to vote Yes is a huge task, but it does create its own overriding focus. The business of actually negotiating the details of independence and then making it work would be much more difficult and stressful. In the months leading up to the reality of independence, the SNP would be tested as never before.

For the time being, the party is showing that, to coin a phrase, it is better to work together.

The Unionists are not united. Their parties claim that they are better together, but the reality is that they are not really together at all, either internally or even in terms of their uneasy and expedient anti-independence campaign. They tell us that they are "together" when they are here in Scotland. The moment they go south of the Border, where most of their voters are, and most of their money comes from, the togetherness disappears.