Geoffrey Wheatcroft: This by-election is a regular fixture: Catholics vs Protestants

Whether or not the Government holds Glasgow East on Thursday, this by-election helps explain why Labour retains a bedrock of support in Scotland. Religious faith may have faded, but sectarian loyalties remain: Glasgow East is a Catholic constituency, and Labour is the Catholic party.

Two centuries ago, Scotland was overwhelmingly a Presbyterian country. All this changed with immigration from Ireland, which had begun before the Great Famine in the 1840s and then became a flood. Most cities in Scotland and England – Glasgow and Liverpool, above all – ac-quired large Irish Catholic populations, leading to a bitter sectarian conflict. Notoriously, this was reflected in football, as well as politics, with Rangers (Protestant) and Celtic (Catholic) in Glasgow, "Hearts" and "Hibs" in Edinburgh, and, in Dundee, Dundee FC and Dundee United.

In England and Wales, the Liberals were the party of Protestant Dissent, until Labour succeeded them. The early Labour movement had deep roots in the chapels, and plenty of men graduated from the pulpit to the hustings. But, in Scotland, something else happened, as Catholics turned to Labour. This was personified by one man, John Wheatley, whose story few know better than Gordon Brown.

In his own hot youth, the Prime Minister wrote an admiring biography of Wheatley's colleague and political pupil, James Maxton, the fiery leader of "Red Clydeside". At the 1922 election, those Independent Labour Party extremists (as even some Labour people saw them) captured 21 of of 28 seats in the Clyde region. Although Maxton was a Presbyterian (like his biographer), several Clydesiders were Catholics, most conspicuously Wheatley, a self-made businessman, born in Co. Waterford before his family migrated to Scotland. He, more than anyone, captured the Catholic vote for Labour, as he moved from the United Irish League by way of the Catholic Socialist Society to the ILP.

As Minister of Health, Wheatley was the most effective member of the short-lived first Labour government of 1924, although still obedient enough to his church, he refused to allow health authorities to give advice on birth control, which foreshadowed a more dramatic episode. During the second Labour government of 1929-31, Sir George Trevelyan's Education Bill was ambushed by Catholic MPs on the Labour back benches, acting on behalf of their bishops.

And that alliance cut two ways electorally: if Labour meant Catholic, then the Conservatives were the Protestant party. Even at the Labour landslide of 1945, the Tories still won a third of the Glasgow seats, thanks to the Orange vote, which explained also why they held a majority of Liverpool's seats in the 1950s. Most startling in hindsight, at the 1955 election, the Tories won a majority of seats in Scotland.

Less than 40 years later, they didn't win a single Scottish seat, and they have remained a negligible force north of the Tweed since the 1997. Religion isn't the only reason for that collapse, but it is a neglected political truth that Scotland, like England, ceased to be a Christian country before it ceased to be a Protestant one – and is now nothing at all. But while Labour hangs on to its residual affiliation – with probably more Labour-voting "Catholics" in Glasgow East than go to Mass – the Tories have been gravely damaged by the eclipse of political Protestantism.

That didn't stop the Tories winning more votes than Labour at the last general election (another neglected fact), and it is almost certain that they will win more English seats at the next. If they fail to gain a parliamentary majority because of Labour's Scottish-Catholic hard core, might not "Orangeism" yet return in new guise?