MPs have been debating whether the UK's nuclear deterrent - Trident - should be renewed. A motion by the SNP, Plaid Cymru and the Greens called for the cancellation of an upgrade to the weapons system. But how important to voters is the debate over nuclear missiles which are carried by submarines based at Faslane naval base on the Clyde?
What are the polls saying?
For the most part, the majority of polls suggest that there is a smallish plurality opposed to the renewal of Trident. However, it depends a bit on how you word the question - for example, a Survation poll for the SNP asked people whether they opposed or supported a new generation of Trident nuclear weapons being based on the River Clyde? Put that way it tends to be rather unpopular.
But if you ask about the principle, do those questioned react differently? A typical example was a poll done by ICM for the Guardian last month in which 43% said they wanted nuclear weapons scrapped; 37% want them maintained. That response is not untypical.
Equally the polls tend to suggest opposition to Trident is somewhat greater in Scotland than it is south of the border. The truth is that on both sides of the border public opinion is pretty seriously divided.
How is the Trident debate relevant to the Scottish independence question?
I think the interesting thing north of the border is the way in which attitudes towards Trident and attitudes towards the constitutional question seemed to have become much more clearly aligned and are, therefore - to some degree at least - reflected in the way some people vote.
If you go back to the Guardian poll, for example, quite astonishingly, it finds that 77% of those people who say they are going to vote for the SNP are opposed to Trident, whereas in contrast only 22% of those willing to vote Labour are opposed to Trident.
That shows Labour having a vote in Scotland that is more in favour of keeping at least some kind of nuclear weapons facility than are opposed.
How much does the Trident debate matter to Scottish Labour?
As far as Scottish Labour leader Jim Murphy is concerned that same Guardian poll identified the views of those people who voted Labour back in 2010, but voted "Yes" in the referendum. It showed that an overwhelming 72% of them were opposed to the renewal of Trident.
So, Labour's problem - north of the border at least - is that this issue of Trident seems to be mixed up with the issue of independence, partly because at some people just don't like nuclear weapons being in the River Clyde.
Therefore Labour has to think if it wants to get back these ex-Labour voters who voted "Yes" and what they say about Trident could actually matter.
How does the Trident question matter to the SNP?
For the most part, the SNP have pretty well soaked up the anti-nuclear vote and it is probably only those who are both anti-nuclear and who want Scotland to be independent who are probably likely to be motivated by this issue.
The interesting thing about the House of Commons debate on the future of Trident is not the event itself but the signal it sends out from what you might call the nationalist group. The SNP, Plaid and the Greens - all of whom say they will negotiate jointly in the event of a hung parliament.
Nicola Sturgeon has said that for her whether or not the next UK government is committed to renewing Trident is a red line issue, The implication is that she would not be willing to support a Labour government, even in terms of "confidence and supply", if it were willing to say it wouldn't get rid of Trident.
How does the Trident question matter to the Greens?
In contrast, leader of the Greens, Natalie Bennett, has very clearly said that she is absolutely opposed to Trident and will vote against it. However, she has implied at least that the Greens don't regard this as such a red line issue - that is, her party might be willing to support a Labour government, but then vote against it on this particular issue.
I think there is an argument that says there now needs to be a bit of clarity within the nationalist grouping to whether a red line issue means not being willing to support Labour in a confidence and supply agreement.
- John Curtice is currently Professor of Politics at the University of Strathclyde. He writes on electoral behaviour and researches political and social attitudes. Prof Curtice's work can be found on the What Scotland Thinks website.