One thing that has surprised pretty much everyone in Westminster over the past couple of years is the endurance of Labour message discipline. There are occasional bursts of sniping from the sidelines – usually in the form of anonymous briefings and half-reported mutterings by sullen MPs when Ed Miliband seems to be under-performing. Then he makes a clever speech, or the government does something idiotic, and everyone falls neatly back into line. It is really quite impressive for a party that was heavily defeated in 2010 and was supposed, according to the pre-ordained media script, to fall apart in civil strife.

The usual explanation for this unity is that Miliband hasn’t made enough tough policy choices that would open up the divisions that were, according to the now yellowing media script, supposed to yawn wide in 2010. That is partly true – and, in the spirit of full disclosure, I should point out that I make that very argument around half-way through this column in the Sunday Times today [2].

But it is also worth noting that discipline doesn’t come exclusively from policy vagueness. Plenty of Labour MPs are unhappy with the leader’s lack of clarity on a range of issues, but they don’t say so on the record. Indeed, one senior BBC journalist complained to me the other day about how hard it was getting Labour guests on a flagship Westminster programme who might say anything at all to deviate from the official Team Miliband script. That isn’t just vagueness. It is strength of will and determination to win.

By contrast, the Tories simply can’t stick to the lines David Cameron would have them parrot. David Jones, the Welsh Secretary, properly disgraced himself [3]last week by suggesting that gay couples can’t bring up children in a nurturing environment. In theory, Tory divisions on gay marriage have been sanctioned by Downing Street – parliament had a free vote on the issue. But that doesn’t excuse cabinet ministers dribbling out tired and hateful homophobic clichés about the unsuitability of gay men or women for loving family relationships.

If Conservatives were at all loyal to Cameron and truly determined to win an election, they might at least pretend to believe some of the same things their leader does.

Then there is Maria Hutchings, the Tory candidate in the Eastleigh by-election, who was reported last week to have said her son’s ambitions to be a surgeon meant he couldn’t possibly attend a state school [4]. That is surely not how Cameron would have phrased his views on comprehensive education. Hutchings also disagrees with Cameron on gay marriage and has said she would vote to leave the European Union. The Daily Mail recently ran a profile of her as the “off-message” candidate [5]- as if fighting the by-election on a Ukip-lite platform were part of some cunning plan by Tory high command to close off the threat from Nigel Farage and thereby snatch the seat. I don’t buy it. Hutchings was the Tory candidate in 2010 – picked from an “A-list” – when the party was supposed to be modernised, socially liberal, Cameroon.

The fact that she is the candidate again (and it is always a hazard offering voters a meal they have sent back to the kitchen once before) proves only that the Cameron operation didn’t really vet it’s A-list very well and hasn’t been paying much attention to candidate selection since getting into government. But now they’re stuck with Hutchings and are doing their best to make a virtue of her. She might still win, of course, but I imagine Cameron would rather be bringing loyal MPs fashioned in his own image into parliament instead of more “off message” mavericks. I doubt Downing Street chose to withdraw the whip from Nadine Dorries only so they could import Eastleigh’s Dorries tribute act.

Then consider John O’Farrell, Labour’s minor celebrity candidate in Eastleigh. A comedian and writer – surely he would be a liability, firing off message all over the place. But no. Even on Twitter, his wit has been channelled carefully down the appropriate slogan-delivery tubes. On 14^th February, he joked:

Fear I have already turned into political robot. Valentines card to wife just said 'Vote Labour in #Eastleigh for a One Nation alternative'.

I suspect that was closer to the truth than he or anyone else in the Labour machine would like to admit. The party is still pretty good at command and control on the ground, especially in a by-election. In one contest recently, I phoned the candidate directly on his mobile to ask what he was up to and was told, quite plainly, that he had no idea but would find out soon enough because his party minders would tell him and steer him in the right direction.

It’s worth noting also how astonishingly disciplined the Lib Dems have been in government, given how cruelly coalition has savaged their opinion poll rating and massacred their local government base. That can’t just be down to party organisation. For one thing, the Lib Dems don’t have enough money to run a ferociously organised party and for another thing, their democratic structures – where everything is supposed to be settled by committee – are positively designed to amplify dissent. As with Labour, the Lib Dem discipline flows from many individual efforts of sheer will. Even if MPs are uncertain about the leader’s line, they stick with it in public.

But for that to work, there has to be a line to take. Whether they agree with Miliband or not, Labour MPs have a pretty clear sense of where he is coming from and what he is trying to do. They understand the “too far, too fast” message on the economy and they broadly understand how “One Nation Labour” is a soft social-democrat fudge to make everyone feel good about the future without making specific commitments to reduce public services in austere times. They may not think that is the best strategy, but they can do it if a microphone is put in front of them.

Likewise, the Lib Dems know the plan is to present themselves as more economically responsible than Labour and more compassionate than the Tories. They know what it is they are supposed to have achieved in government and how it is meant to have tamed the wilder excesses of fanatical Conservative back benchers. They may not think it is enough, but they know the script.

That doesn’t seem to be the case for the Tories. Naturally they can defend government policy when asked to do so but that isn’t the same as defending David Cameron’s policy. He supports gay marriage and very much wants Britain to stay in the European Union, for example. It is hardly news that many Conservatives don’t agree with their leader on certain totemic cultural matters. What I find remarkable is how unwilling they are to keep quiet about it and how rubbish the Number 10 machine is at keeping a lid on unhelpful noise.

The root of the problem – and one explanation for different patterns of behaviour in different parties – is that there are two elements to message discipline. Yes, MPs need to be disciplined, but leaders also need to know what their message actually is to begin with.