Electoral pacts remain rare in UK politics; but they are not new. The first Labour MPs arrived in parliament in 1906 after a pact with the Liberals. During the world wars, the main parties observed non-aggression byelection deals. In 1983 and 1987, Liberals and the Social Democratic party struck a general election seats pact. And there have been notable individual constituency deals too. These range from the Oxford byelection in 1938 over the appeasement of Hitler to the Batley and Spen byelection in 2016 after the murder of Jo Cox. In Northern Ireland, tactical withdrawals to give a nationalist or a unionist a clearer run at the other remain commonplace to this day. A slew of new arrangements of this kind are likely to have a big impact in Northern Ireland’s 18 seats next month.

This week’s announcement of a Unite to Remain pact on 12 December between the Liberal Democrats, the Greens and Plaid Cymru is therefore not unprecedented. It could be significant in three ways: its immediate impact in a number of tightly contested constituencies in England and Wales next month, its consequences for Brexit and, perhaps more lastingly, for the wider signal it sends that traditional partisan politics are under challenge from shifting alliances of less tribal voters. This in turn is partly a result of the transforming impact of Brexit on British party politics and partly of a perception that both Labour and the Conservatives have moved decisively towards the extremes. This has helped to spawn new campaigns such as Common Ground and the all-party More United movement.

There is no getting away from the fact that this week’s pact is only a deal between three minor parties. The SNP and Labour – the former a remain party, the latter a party with a large majority of remain voters – have both refused to join in. The pact involves only 60 seats out of 650, 43 of them in England and 17 in Wales. Its direct impact is likely to be small. While it increases the chances of the Lib Dems in, say, Richmond Park, the Greens in Bristol West and Plaid in Ynys Môn, all three of which are among the respective parties’ targets, it still leaves the Greens and Plaid a long way from translating the pact into more seats at Westminster. If the pact delivers, the Liberal Democrats stand to be the disproportionate beneficiaries. Even so, in a tight election even a small number of Conservative losses in seats the party might have expected to win in the absence of a pact could be vital.

The other 572 seats in Britain – the overwhelming majority – will not be directly affected by this week’s pact. True, a small number, including Beaconsfield, Broxtowe, Calder Valley and Luton South, have already witnessed unilateral Lib Dem or Green withdrawals in order to help pro-remain candidates, including pro-remain Labour ones. True also, there are likely to be informal decisions by Labour not to contest some Lib Dem target seats very strongly; perhaps even some formal decisions too, as advocated by Clive Lewis or Paul Mason. But the main effect of the pact in these seats is likely to be through osmosis and as a result of anti-Brexit tactical voting campaigns.

This week the Labour deputy leader Tom Watson joined the increasingly long list of moderately inclined Labour and Conservative MPs stepping down from politics. His departure, and those of many recent Tory ministers, seem to confirm that the big parties are not interested in working across even their own divides, never mind those between them. This week’s pact is a reaction against the imperfect choices on Brexit from the two main parties. But in another contest the pact might easily be a cross-party response to the failure to adopt radical and agreed measures to combat the climate crisis.

Electoral pacts are a consequence of the first-past-the-post voting system. If Britain’s electoral system was fairer, and less prone to confer majoritarian powers on parties with only minority support among voters, they would not be necessary. As long as the voting system is unreformed, the pressure for pacts will be there, as it is today. But pressure to cooperate in the national interest stems from policy failure too. The system is wrong, but so is the political direction.