Could Britain's 2015 vote be upended by Scottish referendum's ripples?

Just a fortnight after the British union survived a referendum on Scottish independence, the focus has turned squarely on what is expected to be a tight general election next May.

Campaigns have already started in earnest. Last week, the opposition Labour party set its stall out, promising a fairer society and a reduction in the cost of living. Earlier this week, Prime Minister David Cameron pledged tax cuts if his Conservatives win.

But the concluded Scottish referendum may still influence who ends up with the keys to 10 Downing Street in London, thanks to a surge in political engagement not seen in Britain since the 1990s. In less than two weeks, tens of thousands of Scots have joined pro-independence political parties – bringing a new uncertainty to the region's support for Labour, which has relied on Scottish backing for decades.

Since September’s "no" vote, the Scottish National Party (SNP) has seen its membership surge from around 25,000 to more than 75,000, making it the third largest political party in the whole of Britain, behind only Labour and the Conservatives. The pro-independence Scottish Greens have more than tripled in size since the Sept. 18 vote, while the smaller Scottish Socialist Party has also experienced record growth.

That puts the SNP on course to win a record third successive term in the next devolved Scottish elections. But more importantly for Britain next year, some commentators suggest that Scottish nationalists could win enough votes in the general election to deny Labour victory.

A no-longer-split vote?

Since devolution in 1997, the Scottish electorate has regularly shifted between parties depending on the election being fought. In recent years, Scots have tended to vote for Labour in Britain's national Parliament in Westminster, but for Scottish nationalists in the devolved parliament in Holyrood.

But if Scots were to vote in large numbers for the SNP in Westminster too, it could have a significant effect on the electoral arithmetic across Britain, making it almost impossible for Labour to secure an overall majority.

"Parity in votes between SNP and Labour next May now looks perfectly possible. This could give the SNP up to 20 more seats, mainly at Labour’s expense," Peter Kellner, president of respected pollster YouGov, wrote on YouGov's website. That would boost Mr. Cameron's Conservatives in their efforts to retain power in Westminster.

The risk to Labour increases if Westminster parties fail to deliver on promises of more devolution for Scotland. "The London-based parties have now offered Edinburgh more powers. These will need legislation in the new Parliament after next May. This gives the SNP a perfect election platform: ‘Vote SNP to force London to keep its promises,’" Mr. Kellner added.

Lessons of history

But others caution that the surge in pro-independence membership rolls does not mean that Scots will break their pattern of voting SNP for Holyrood and Labour for Westminster.

"Until the SNP are actually ahead of Labour in Westminster voting intentions, they don’t pick up a great deal of seats," says John Curtice, professor of politics at the University of Strathclyde in Glasgow. He notes that that while 45 percent of Scots voted SNP in the 2011 Scottish elections, just 20 percent did so in the previous year’s British vote. 

Moreover, Professor Curtice adds, recent history has shown that spikes in British party membership tend to normalize quickly. "The last time you had a spike in party membership [anywhere in Britain] was New Labour in the 1990s," he says, referring to a tide of support for Labour under Tony Blair, "and that didn’t last."

"You have to see whether these people stay," he says. "Labour may lose a few [Scottish] seats in Westminster, but being decimated? Hardly."