You scratch my back, I'll scratch yours' is often a powerful glue in politics. Yet it seems that it is a glue to which, in recent weeks, many a Tory MP has been oblivious - to their cost.

Constitutional reform always looked like a potentially fraught area for any coalition deal between the Conservatives and the Liberal Democrats. The former have always tended to be cool on changing the way we are governed, while the latter have long been enthusiasts. Yet two years ago they managed to find the basis of a deal.

In opposition, both parties had backed a populist cut in the number of MPs, appealing to the anti-politics mood that had been fuelled by the expenses scandal. At the same time, the Tories wanted to change the way in which constituency boundaries are drawn up, in the belief that this would overcome the adverse way in which the first-past-the-post system is currently treating them. To that proposal the Liberal Democrats acquiesced, while securing the possibility of a switch to the alternative vote for Commons elections - so long as voters agreed - together with the prospect of a House of Lords elected by proportional representation (two reforms the party could expect to profit from).

However, last year voters rejected the alternative vote, not least thanks to the campaigning efforts of David Cameron and his colleagues. So when, last week, Nick Clegg was told that Lords reform is off the agenda too, the spoils of office for the Liberal Democrats had all but disappeared.

That might seem reason enough for them to pull the plug on the boundary review and deny the Tories their spoils too. But in truth the impetus for the Liberal Democrats' decision to vote against the results of the review when they are laid before parliament - in the knowledge that they are likely to be joined in the 'No' lobby by Labour - goes beyond simple tit for tat. The party has every good reason to want to see the back of the boundary review.

It was probably never wise for the Liberal Democrats to have promoted cutting the number of MPs. As a third party with a relatively evenly spread vote, it struggles to get the local concentrations of support needed to turn votes into seats under first-past-the-post. Cutting the number of MPs means increasing the size of constituencies, thereby making it more likely that whatever local concentrations of support the party does manage to build up are diluted away.

Thus, when the first estimates of the likely impact of the boundary review on party fortunes came out, it was little surprise that it was the Liberal Democrats that looked set to lose the most. According to Anthony Wells of, if the provisional new boundaries had been in place in 2010 they would have resulted in a 20 per cent cut in Lib-Dem representation, compared with a 10 per cent drop for Labour, while the Tories would hardy lose any seats at all.

Meanwhile, the Liberal Democrats have been suffering terribly at the ballot box, as voters flee the party in dismay at the party's role in the coalition. On average, it is barely managing to register double figures in the polls. In combination with a boundary review, such a performance could mean the party would be reduced in 2015 to a rump of just half a dozen MPs, its worst result for over 40 years.

In scuppering the Liberal Democrats' hopes of securing a proportionally elected Lords, Tory MPs have thrown Nick Clegg and his colleagues a lifeline. Sitting at 10 per cent in the polls, a dozen of them might manage to survive in 2015 - twice as many as would do if the boundary review were to take place. Liberal Democrat turkeys have gratefully seized an unexpected opportunity to vote for the postponement of Christmas.

At the same time, Tory MPs have also potentially put at jeopardy their own party's future prospects for power. With the boundary review in place, the seven-point lead over Labour the Tories secured in 2010 might just have been enough to secure a narrow overall majority. Without it, then, on the conventional calculations, the party will need no less than an 11-point lead - not just one more heave but a landslide. With the party now finally beginning to suffer mid-term blues, achieving such a result has come to look like a bit of a tall order.

Meanwhile, if the Tories do find themselves short of a majority in 2015, the chances of the two coalition partners striking another deal have probably been significantly reduced. Liberal Democrats may decide there is little to be gained by scuttling this coalition now, but after their experience of making no headway on constitutional reform in this parliament they are likely to be very reluctant indeed to get into bed with the Tories once again. Is it time, perhaps, for Labour to begin to make its own moves on constitutional reform?