If the Conservatives are trying to fix the voting system it could backfire on them

The Conservatives are accused of changing the voting rules in ways that will favour them. Two changes are proposed: one is to require people to show proof of identity before they vote; the other is to abolish preferential voting and to use first-past-the-post instead for mayoral and police and crime commissioner elections. Opposition parties furiously accuse the Tories of seeking party advantage.

But the evidence suggests that neither change would benefit the Conservative Party. Requiring voter ID is likely to cause problems for groups that are less likely to have ID, but old people are the largest group in that category, and they tend to vote Conservative. Young people and ethnic minorities are also more likely to be affected, and tend to vote Labour, but the overall partisan effect is unclear.

This doesn’t mean that voter ID law is a good idea – it is certainly a solution to a non-existent problem – but it is unlikely to “suppress” the Labour vote relative to the Tory vote. By importing the US term “voter suppression”, opposition politicians are misleading themselves.

The other change that looks as if it is motivated by party advantage is the plan to abolish the “supplementary vote”. This is the system that was used last month to elect mayors and police and crime commissioners (PCCs), which allows voters to cast a second-preference vote if no candidate wins 50 per cent of first-preference votes.

Priti Patel, the home secretary, has announced that the government intends these elections to use first-past-the-post in future – voters will have a single vote and the candidate with the most votes wins, even if they have less than 50 per cent of the total. It might look as if this were a response to results such as the Cambridgeshire and Peterborough mayoral election, in which James Palmer, the Conservative incumbent, won the most first-preference votes but was beaten by Nik Johnson, the Labour candidate, who won the most second-preference votes when the third-placed Liberal Democrat was eliminated.

Indeed, a survey by David Cowling, the former BBC elections analyst, of all mayoral and PCC elections since they were introduced in 2000 found that the Conservatives lost out from the supplementary vote more often than Labour.

Of the 218 contests where the supplementary vote was used, in 17 (8 per cent) a second-placed candidate in the first round of counting leapfrogged into first place. Independent candidates benefited most, on 11 occasions, and lost out on only two, making a net gain of nine from the supplementary vote system. Labour gained three and lost seven, a net loss of four. The Conservatives gained two and lost eight, a net loss of six. But there is not much in it, and the best known occasion when the supplementary vote changed the outcome was when John Prescott, the Labour former deputy prime minister, stood to be Humberside PCC in 2012 and was defeated on second preferences by Matthew Grove, the Tory.

So the record shows that, although the Tories have lost out from the supplementary vote, Labour has lost out almost as much. If the Tories are changing the system for the sake of party advantage it is because they want to curb the success of independent candidates – not because it would hurt Labour, their principal national opponents.

Bringing in the voter ID requirement and abolishing the supplementary vote may be good or bad ideas, therefore, but if they are motivated by party-political advantage, the Conservatives are set to be disappointed.