An analysis of economic influences on political popularity suggests that the Conservatives will move ahead of Labour in the polls by the time of the May 2015 general election but their lead will be insufficient to deliver a majority in the House of Commons, and may not even result in the party holding the most seats in a hung parliament.

Statistical analysis of Guardian-ICM poll results since 1984 shows that the voting intentions differential between the main governing and opposition parties depends positively on average earnings growth and house price inflation, and negatively on the unemployment rate, retail price inflation and interest rates (Bank rate). The table reports the estimated sensitivities of the differential (i.e. Conservatives minus Labour currently) to these factors. A rise in headline average earnings growth, for example, from the current 1.7% to 2.7% would boost the Conservatives relative to Labour by 4 percentage points. An increase in Bank rate from 0.5% to 1.5% would have the opposite effect.

The model from which these sensitivities are derived also allows for “honeymoon” effects enjoyed by newly-elected governments – larger when the election results in a change of control – and a similar goodwill boost when there is a mid-term change of prime minister (Thatcher/Major in 1990 and Blair/Brown in 2007). The chart below shows the history of the poll differential between the main governing and opposition parties and the “prediction” of the model. While there are periodic significant divergences, it is, perhaps, surprising how much of the variation in polls can be explained by economic factors and honeymoon effects.

The Guardian-ICM poll published this week reports a Conservative-Labour differential of -5 percentage points, which is in line with the current model estimate. Election “swingometers” designed to translate voting intentions into seat predictions indicate that this would deliver a sizeable Labour majority. The next chart, derived from the swingometer on the Electoral Calculus website, shows required Conservative-Labour differentials that would result in majorities for either party for different levels of support for the Liberal Democrats, assuming that UKIP and smaller parties account for 10% and 5% of the vote respectively*. If the Lib Dems maintain their 12% showing in the latest Guardian-ICM poll, the suggestion is that the Conservatives need to open up a lead of about 6 percentage points over Labour to achieve a majority, and to be 3 points ahead to hold the most seats in a hung parliament, implying a first shot at forming a new coalition government.

Economic trends are moving in the Conservatives’ favour but will they deliver the necessary turnaround in the polls? The model can be used to examine this issue. The first scenario considered assumes that the economy performs in line with the Bank of England’s latest forecast**. The Bank expects average earnings growth to rise to 2.75% by the fourth quarter of 2014 and 3.75% by end-2015. The unemployment rate drops from the current 6.9% to 6.5% by the time of the election, while inflation is broadly stable. The forecast incorporates the market expectation that overnight interest rates will rise to 0.8% in the second quarter of 2015, implying a quarter-point increase in Bank rate. The Bank does not publish its projection for house prices – these are assumed to rise by 10% per annum. On these assumptions, the model estimates that the Conservatives would move into a 1.5 percentage point lead by May 2015, i.e. insufficient even for them to gain a plurality of seats in a hung parliament – see chart below.

The judgement here is that the economy will perform more strongly than the Bank expects, leading to a more rapid fall in unemployment and greater upward pressure on earnings. The second scenario considered assumes that earnings growth rises to 4% by the time of the election, while the unemployment rate drops to 5.7%. An additional difference is that Bank rate is held at 0.5% until after the election. The other assumptions – i.e. for inflation and house prices – are unchanged. According to the model, this “best case” scenario for the Conservatives would give them a lead of about 6 percentage points over Labour by May 2015, suggesting that they could scrape home with a single-figure seat majority if Lib Dem and UKIP support remains at current levels.

The problem, of course, is that the scenario is internally inconsistent – the strong pick-up in wage growth would be likely to push inflation higher and force the Bank of England to raise interest rates. A final scenario was considered maintaining the “best case” assumptions for earnings and unemployment but in which retail price inflation rises from the current 2.5% to 3%, while Bank rate is increased in two quarter-point steps to 1.0% by May 2015, with this leading to a slowdown in house prices. The Conservative-Labour lead is reduced to 2.5 percentage points, suggesting that the two parties would hold similar numbers of seats in a hung parliament.

Summing up, the modest impact of better economic news on Conservative support is understandable, because strong growth has only recently started to feed through to a recovery in real earnings. Faster pay expansion and a further unemployment decline could lift the Conservative-Labour differential by as much as 10 percentage points over the next 12 months. The associated economic scenario, however, would warrant a rise in interest rates, which would probably reverse part of the Tory gain. The outcome of the 2015 election will depend importantly on whether the Bank of England sets policy purely on economic grounds or delays necessary action to avoid overt influence on a close political race.

*Increasing the UKIP percentage to 15% has only a minor effect on the results.
**February Inflation Report forecast based on market-implied interest rates.