I was once told by someone in politics – I forget who – that governments pass through three basic phases. The first is when they are new, and they can blame everything that goes wrong on the last lot in power. The second is after a couple of years, when they have to take the blame themselves. The third is when the next general election approaches, and they have to tell voters why they’re the best pick for the future.

David Cameron was fortunate during the last Parliament, in that the first of these phases could be stretched out for almost the whole five years, then blended in with the third. The sheer catastrophe of the economic crash left Labour vulnerable for longer. The general election result suggests that they never really recovered.

But what about now? Judging by this morning’s newspapers, Cameron is trying to restart the lifecycle by returning to the first phase – and blaming everything that goes wrong on the last lot in power. Except, in this case, the last lot in power weren’t Labour. They were the Conservatives in coalition with the Liberal Democrats.

This report in the Times (£) contains all of the relevant quotes. “In terms of doing things differently,” Cameron told journalists yesterday, “I think it does help when we’ve got one person at the steering wheel driving in consistently the same direction.” He went on to describe the frustrations of coalition, including “having fiefdoms perhaps run by a different party”. Majority government will help, apparently, to improve “our performance on immigration”.

You can understand why the Prime Minister is saying all this. Now that the Conservatives and the Liberal Democrats have divorced, with one side losing almost everything, he is practically duty-bound to reassure us that one partner is better than two. He may yet be proved right.

But he still ought to be careful, not least for the reasons I have outlined in some of my recent posts about Tim Farron. (Yes, I know, I do rather go on about this stuff.) As I wrote a week ago, it is “both impolite and potentially imprudent” for the Conservative leadership to defame the Liberal Democrats now. Impolite because, without support from Nick Clegg and his party, they may not have been able to do any governing at all. Imprudent because a 12-seat majority is a fragile construct indeed. The Lib Dems might have to be called on again.

Yet there are other reasons for Cameron to guard his mouth. The shift from the first to the second phase of government comes about, my informant said, because voters start to tire of the blame game. The emphasis on the previous lot comes to be seen for what it so often is: a lazy excuse. And that process could be accelerated this time around. With the Cables and Daveys and Lawses now out of Parliament, the public’s memory of them will soon fade towards naught. They will be left remembering that Cameron has been Prime Minister for five, six, seven years and counting.

And then there is the matter of positivity. Matthew Parris writes in his latest column that the Conservatives are in an almost unprecedented position to “enthuse and inspire about the future”. A sour-faced concentration on the past, even the recent past, will squander that opportunity.