More than a third of the voters here declared themselves Catholics in the last census, making it one of the most Roman areas of the country. Now Son of the Manse Gordon Brown is about to have his destiny decided largely by the members of this sometimes obstreperous clan – a fact which last week was used to explain why Brown had postponed a vote on the contentious issue of embryology research.

Of course, like most people, faith does not top the list for most Catholics when they consider their priorities before voting. They are more likely to consult their bank statements than Humanae Vitae before heading off to vote. The caricature of the Joycean priest browbeating the faithful into the voting booth is long gone, even if some of the Liverpudlian clerics who reputedly used to stand outside polling stations demanding parishioners voted Labour may still be alive.

Nevertheless, speaking as a left-footer myself, and as a former scribe for a Catholic newspaper, I suspect the average Catholic does pay more regard towards their religion than members of other churches. Considering the volubility of the Catholic Church's social and moral stance in the wider world, and the effects of Catholic education, that simply stands to reason.

The Labour Party has been the most significant beneficiary of this. Polling research by Mori after the 2005 election concluded that had all Catholics stayed at home on voting day, Tony Blair would have lost. Support among Catholics for Labour stood at a remarkable 53%, miles ahead of the 36% which Blair secured across the population as a whole. Blair has since converted to Catholicism; we all like to be among friends.

In Scotland, Professor John Curtice of Strathclyde University has conducted research showing that Catholics still broadly back Labour. On the whole, Catholics are more left-wing than the average Scot. Again, this stands to reason: while the Catholic Church's public front is that of the fuming cardinal or bishop, most Catholics experience their church more as about social justice than heavy morality, more Parable of the Good Shepherd than Congregation for the Doctrine of the Faith. It is easy to see why Labour, with its message of opportunity for all, hoovers up support.

But, of course, this only factors in one half of the Catholic brain. And it is the other side which Labour can't handle. Here are good people, notes the Labour politician, who appear to agree with him on the issues that led to the creation of his party: social welfare; the moral necessity to help the poor; the need to redistribute wealth. Here are people who come up with phrases such as the "option for the poor", and "the common good" which – he notes – could easily have featured in his own manifesto.

And then, the Labour man sighs, throwing his hands up in the air, Catholics lose the plot. When you press the need to give children a good grounding in sex education, they accuse you of "state-sponsored child abuse". When you attempt to reverse a law banning the promotion of homosexuality in the classroom, they describe it as a "perversion". When you try to encourage research on diseases by allowing hybrid embryos, they call you "Frankenstein".

It is this right-side of the Catholic brain which currently shouts the loudest. Catholic composer James MacMillan penned an article last week in which he predicted that Labour would lose the Glasgow East by-election because of its betrayal of traditional morality, citing the party's refusal to tighten abortion laws, the creation of "saviour siblings" and its backing of hybrid embryos as evidence. "Christian beliefs are being dumped contemptuously by the Labour Party," he declared.

Several Catholic Bishops in Scotland have joined in the attack, prompted by an increasingly irritable Vatican, which sees Europe – "its" Europe, the very place where the Church was built – descending into a post-modern secular gloop.

It is a moot point as to whether the moral conservative sensibility within the average Catholic will begin to hold sway over the socially liberal side. But for the moral conservatives, they know they have an option. And here Alex Salmond's own moral conservatism heaves into view.

The First Minister voted to cut the abortion limit earlier this summer and he shows few signs of wanting to provoke the Christian right. In a recent issue of the Scottish Catholic Observer, Salmond declared he would do everything he could to ensure Scotland's two Catholic adoption agencies would be exempt from new equality laws which could compel them to take on homosexuals as prospective parents. The newspaper's editorial this week notes its guarded approval. The old Catholic suspicion of the SNP – that independence would, in effect, create a second Ulster – is not even mentioned. The SNP brand, to use the current parlance, is increasingly "detoxified".

Last week, various pundits were wheeled out to insist that there was no longer such a thing as a "Catholic vote". Really? I suspect that they are anti-religionists who wish the world was completely secular, with no troublesome Catholics (or Protestants, or Muslims). There assuredly is a Catholic vote and it will feature in Glasgow East. What is fascinating is that we can no longer really be sure where it will go.