Onward Christian politics, marching to the Holyrood elections. Bishop Joseph Devine, one of Scotland's leading Catholics, has put the religious cat among the Labour Party pigeons when he said that he would not vote in protest at the new law on gay adoption.

Catholic priests are banned by their Church from getting involved in party politics, but the Bishop of Motherwell has promised to endorse another party once he has read their manifestos. He declared the Christian People's Alliance (CPA) as front runner for now.

How worried should Labour be? The evidence is unclear. If the past behaviour of politicians is any guide, Labour gets very anxious when the Catholic hierarchy criticises it or talks positively about independence.

In 1979, 67% of the Catholic vote went Labour, while only 7% went SNP. By 1992, that had declined to 53% Labour, with 16% for the SNP. Since then, Alex Salmond has courted Catholic voters with hearty support for separate schooling and repeal of the discriminatory Act of Settlement.

Professor James Mitchell, of Strathclyde University, argues Catholics' pro-Labour habit is less about religion than because the party supported Irish immigrants, and that all such voter blocks are now becoming less predictable and loyal.

"It is easy to overstate the role of religion in politics today, but it is also easy to understate its importance in the past," observes the academic.

The next question is whether the hierarchy can sway the votes of church adherents.

There is little evidence it does. Catholic churches have given prominence to Pro-Life candidates' campaigns, but they have failed dismally. The pressure exerted by the Church over the repeal of Section 28, the law on teaching about homosexuality in schools, made no discernible impact.

Catholics and those from other denominations now say the law allowing gay adoption is mobilising Christians, and that is where the potential for other smaller parties comes in.

The Scottish Christian Party (SCP) is led by the Rev George Hargreaves, a Londoner of Afro-Caribbean origin and former pop song lyricist. Based at his St Andrews home, it borrows from American evangelicals' political campaign tactics. It is hoping to defeat the Greens' ultra-liberal Glasgow MSP Patrick Harvie and to build on a strong performance in the Western Isles in the 2005 Westminster election, when its vote share is credited with doing enough damage to Labour to let the SNP gain the seat.

The party given a tentative endorsement by Bishop Devine, the CPA, sees George Hargreaves as an unwelcome southern import. "I can't stress enough that we have nothing in common with these people," according to Teresa Smith, former Labour constituency chair in Dumfries and now leader of the CPA.

She positions her party with other northern European Christian Democrat parties, in Holland and Scandanavia. The policies are pro-family and pro-life.

Prof Mitchell doubts this year will be as good for smaller parties as 2003, but he points out Britain is out of kilter with the rest of Europe in keeping such distance between church and party. "The absence of a Christian or religious party is unusual," he says.