BRITAIN’S politicians took a break from the beach volleyball this week to do something truly silly. David Cameron, the Conservative prime minister, and Nick Clegg, his Liberal Democrat deputy, in effect killed off two beneficial constitutional reforms. In doing so, the pair weakened the government and greatly harmed both their parties’ political prospects. If they lose power at the next election, the Tories could well wonder what possessed them in the summer of 2012.

The starting point was a government bill that would have made the House of Lords smaller and largely elected. Liberal Democrats are keen on this reform. Many Conservative MPs loathe it, partly because the Lib Dems are so enthusiastic, partly because they suspect an elected Lords would challenge the primacy of the House of Commons. In May many Tory backbenchers, including some who are generally placid, rebelled by voting against the bill. Concluding that it is impossible to win them over, the prime minister has backed down.

Mr Clegg promptly stamped his foot, declaring that the failure of Lords reform represented a breaking of the coalition agreement signed in 2010. He was therefore free to oppose another reform, to electoral boundaries. This would have ensured that all constituencies contained about the same number of voters and would have trimmed the number of MPs from 650 to 600. Thanks to electoral maths, it would probably have hurt the Liberal Democrats most.

So there are excuses and pretexts. But in the long term this spat could prove deeply destructive on three different levels. First, it is sad that constitutional reform has been curtailed. The House of Lords should be elected: it is absurd to have political placemen shaping the laws of the land, let alone people who owe their seats to their family tree. It is likewise right to reshape constituency boundaries to reflect population changes and to create a leaner and cheaper Commons. Votes should count the same across the land. True, the British public has more to worry about than either Lords reform or boundary changes. But the reforms were still broadly useful.

Second, the coalition is now far weaker. By trying to defuse rebellions in their own parties, the Conservative and Liberal Democrat leaders have primed a much bigger bomb under the government. The coalition was once bound together by affinity, with the two parties discovering they agreed on a good deal, including school reform, localism and paying off the deficit. It is now held together more by a fear of what would happen if it dissolved: the Tories trail Labour in the polls, and the Liberal Democrats are floundering. This week may come to be seen as the point at which their pact began to fall apart. By describing the coalition agreement as a purely contractual affair, Mr Clegg inaugurated a new era of tit-for-tat politicking. The project looks less like a marriage and more like a bad-tempered game of chess. That is not in Britain’s interests.

Well done: you saved the Lords and lost the election

The third level concerns the Conservatives—and the damage to their election hopes. The proposed changes to constituency boundaries were intended to correct a strong bias in the electoral system in favour of the Labour Party. In 1992 John Major won a Conservative parliamentary majority of just 21 with a lead of 7.5 points. In 2005, by contrast, Tony Blair achieved a majority of 65 with a lead of just 2.8 points over the Tories. At the next election, set for 2015, boundary reform is thought to be worth between 12 and 20 seats to the Conservatives, who are currently 21 short of an absolute majority. With the Tories all but driven out of Scotland and the big cities, Labour already had a strong defensive position. Now the odds of Labour winning, or sharing power with the Lib Dems, have increased.

The Tory rebels who voted against Lords reform knew their partners would retaliate. If they wake up after the next election to discover Labour in power or in talks with the Lib Dems, and their own party facing five years in opposition, those Tories might reconsider whether keeping an undemocratic Lords was such a great political prize. In the meantime they have made governing harder. Nice work, folks.