With a year to go, only one thing about the 2015 general election is certain: it will demolish old political certainties

NATE SILVER is probably the world’s only rock-star statistician. Using a model based on historical precedents, the American blogger predicted the result of the 2012 presidential election with startling accuracy. All 50 states voted as he said they would. British journalists have since challenged Mr Silver to do the same for Britain’s next parliamentary election, which is due to take place on May 7th 2015. So far he has resisted their goading.

That is wise. With the election a year away, Westminster is palpably shifting into campaign mode: party staff are being reorganised, advertising schedules planned and foreign gurus flown in. But the range of variables and possible outcomes looks unusually perplexing. The only certainty is that some of the established rules of British politics will be broken in 2015. That is causing headaches for psephologists—and party leaders.

Polling suggests that if the election were held now the opposition Labour Party would win a modest majority: it is about five points ahead. Some past patterns indicate that it ought to win next year, too. For decades Labour votes have been more efficiently distributed than Conservative ones. In 2005 a 35% share gave Labour a 64-seat majority. In 2010 the Tories won 36% of votes but were 20 seats short of a majority. This bias looks unlikely to disappear.

Another established pattern is still worse for the Conservatives: no serving government has increased its share of the vote in any of the past eight elections (see first chart). On average, incumbent parties have lost 3.7 points. Governing has political costs, perhaps even heftier now than in the past as a result of austerity. A Labour victory may seem all but inevitable. Yet similarly sturdy precedent suggests that just the opposite will happen. In seven of the eight previous general elections the main opposition party did worse—by 4.8 points on average—than polls had suggested a year beforehand (see second chart). The recent decline in Labour’s lead may be a sign that the party will suffer the same fate. If so, a second Conservative-led government becomes more likely. Other patterns point to a Tory victory, too. It has been four decades since a party last regained office only one term after losing it, as Labour hopes to do. No opposition party has won power without either a more popular leader or a better reputation for economic management than the government of the day. Labour has neither. And, adds Peter Kellner, a pollster, no opposition in the past half-century has succeeded without at least once chalking up a poll lead of at least 20 points. Labour has never led by more than 16 points. The outcome for the two main parties also depends on their smaller rivals. Labour is tussling for left-wing votes with the beleaguered Liberal Democrats. The populist UK Independence Party (UKIP), which particularly threatens Tory prospects, could come first in the European elections in late May. Political precedents are similarly unhelpful guides to how these parties will fare. The Lib Dems are in power for the first time in 80 years, and UKIP’s sudden rise has no obvious parallel.

Why are the old political rules crumbling? Robert Ford of Manchester University argues that there are simply more sources of uncertainty. Britain has its first post-war coalition government. The rise of UKIP, as well as increased non-voting, means that old models predicated on an electorate overwhelmingly supportive of the two main parties no longer work. The familiar notion of a uniform national swing also looks dated. By fixing the parliamentary term at five years, the coalition has given MPs the time to build strong personal followings in their constituencies. If, in September, Scots vote to leave the United Kingdom, the political arithmetic will change even more dramatically.

This uncertainty makes life difficult for the main party leaders. Each is quietly planning for violently different outcomes: outright defeat, coalition talks or (in the case of Labour and the Conservatives) majority government. Each is struggling to hush distracting speculation about post-election negotiations and even leadership contests. Each is torn between a bearish, defensive campaign strategy and a bullish, offensive one.

Doubt also afflicts academics and pollsters attempting to forecast the results. In October Steve Fisher of Oxford University published a mathematical model that involved mapping current polling onto past patterns. Within months he had published a revised model: polls had not adhered to the original one. By comparison with past elections, he notes, the range of conceivable outcomes is “enormous”.

Some are already doing nicely out of the unpredictable 2015 election, though. Turnover from political betting is over four times what it was at the same point in the last parliament, reports David Williams of Ladbrokes, a bookmaker. With so much of what Mr Ford calls “Nate Silver-ism” (amateurs keenly concocting models and counter-models) many are putting money behind their calculations. All bets are off? Quite the opposite.

Correction: Mr Fisher points out that he changed his model in order to better reflect past patterns, not in response to changing poll numbers. His full comments are here.