ARE coalition governments good for reform? According to an argument advanced by centrist Conservatives since the 2010 general election, David Cameron was lucky to find himself forced to rely on the votes of 57 Liberal Democrat MPs to form a government. Given the daunting tasks facing any British government taking office in 2010, their theory runs, he was doubly fortunate to be able to form something approaching a government of national unity.

First, they say, he need not rely on the votes of his hard-right, who might have led him into debilitating rows with Europe or divisive culture wars, undoing his hard work to detoxify the Tory brand. Second, when times are hard and tough policies are needed, modernisers argue that a coalition government is structurally a better vehicle for promoting radical reform—because voters can see two parties from different political traditions thrashing out policies together, they are more willing to accept that the coalition's bolder plans are in the national interest and are not just a plot by “nasty” Tories.

That argument may mightily provoke the Tory right, but it is not wholly unconvincing. Certainly, the coalition has been remarkably successful in convincing a broad mass of voters that Labour left behind a disastrous financial mess that needed urgent fixing. It might have been harder for Tories on their own to make that argument without being accused of yearning to shrink the state.

But if coalitions enable bold reforms, what about the NHS bill currently being paused and watered down at the behest of the Liberal Democrats (even though Lib Dem MPs had voted for the reform plans on their first trundle through Parliament)? Talk to senior Lib Dems and they are quite open that they are grabbing a political prize by presenting themselves as the "nice" party saving the beloved NHS from the "nasty" Tory party. They need a boost after their drubbing in elections on May 5th, they say. And with the Tories always vulnerable on health, they have decided to water down the pro-market reforms being proposed by Andrew Lansley, the health secretary.

Leave to one side the merits of Mr Lansley's plans: a complicated subject which needs its own reporting.

There seems little doubt that the government is preparing a big retreat over its flagship reform plan for the NHS. Tory nervousness about the details of this big and rather confusing reform plan doubtless explains some of the new timidity.

But the scale and pace of the retreat has undoubtedly been greatly increased by the political realities of two-party government. The Tories could see extraordinary risks, and the Lib Dems big gains, from a situation in which voters began comparing the instincts and declarations of the two parties when it came to health policy.

There has been a lot of commentary written this week about how the coalition is still in great shape and the NHS fight has been partly stage-managed, because David Cameron and the Lib Dem leader Nick Clegg are not very far apart on NHS reforms.

That may be so, but I wonder if many voters have picked up that subtlety. I think that voters who keep half an eye on political headlines have seen two big things in recent weeks. During the unexpectedly ugly campaign around the May 5th referendum on whether to keep or change Britain's electoral rules, they saw proxies for the Conservative Party arguing that Mr Clegg and the Lib Dems break their promises and cannot be trusted, and suggesting that increased Lib Dem clout was a key reason to reject any move to the alternative vote.

And now, they see Lib Dems (including Mr Clegg) declaring that the public have a right to be confused and alarmed when they hear politicians talking about their love of the NHS, when at the same time people calling themselves government advisers have been talking about the fat profits to be derived from increasing competition in the NHS.

This strikes me as a big deal. Attacking Mr Clegg on trust, and attacking Mr Cameron's sincerity over the NHS, amounts to direct, deliberate strikes on the core political identities of both men.

I have no doubt that both parties now plan to go back to working together peacefully and productively. But we have now learned that when both sides sense a big enough political prize, they are prepared to go for each other's throats. And when that happens, the benign coalition dynamic (here we are, coming together and governing in the national interest) falls apart. Instead, the public sees two parties who are working together (and thus presumably have the inside dope on what the other is up to) signalling that their partner cannot be trusted. At when that negative coalition dynamic is in play, it is structurally bound to generate timid, watered-down reforms.

Already, senior sources are saying that from now on the coalition is going to be more businesslike, and less trusting. From now on, I am told, if party leaders want to promote fresh reforms dear to one party, they will have to decide whether they want to commit serious political capital. If Mr Cameron wants directly-elected mayors, for example, he will have to tell Mr Clegg that this matters enough to him to commit lots of political capital to it. On the other side, sources tell me, if for instance Mr Clegg really wants to push House of Lords reform (and that is a big "if"), he will have to invest a great deal of political capital, and inform the Tories that this is a reform for which his party will die in a ditch.

That sounds like a recipe for a much slower pace of reform, I suggested to one source. Yes, he replied.

Here is my print column for this week, which looks at these issues:

IN OLD-fashioned zoos, animals that mauled a keeper were taken away and shot. Once a furry inmate acquires a taste for human flesh, the theory went, it can never be trusted again. The junior partners in Britain's coalition government, the Liberal Democrats, have just taken their first big bite out of the Conservatives, demanding that a plan to reform the National Health Service (NHS) be delayed and stripped of its boldest elements.

Plenty of Conservative MPs would dispatch the Lib Dems now if they could. Many never trusted their coalition partners, and Lib Dem attacks on the NHS plans—notably their calls to slow and dilute reforms promoting market-based competition—have confirmed their worst suspicions.

The scrap leaves Tory centrists on the defensive. Modernisers around David Cameron have spent the past year arguing that, when times are hard and tough policies are needed, a coalition government is a better vehicle for promoting radical reform than a purely Tory government would have been, especially one hobbled by a narrow parliamentary majority. As modernisers tell it, because voters can see two parties from different political traditions thrashing out policies together, they are more willing to accept that the coalition's bolder plans are in the national interest, and are not just a plot by “nasty” Tories.

Those arguments in favour of coalition rule now face a stern test. Senior allies of Nick Clegg, the deputy prime minister and Lib Dem leader, are remarkably candid about the three-part process that led up to their mauling of an NHS reform bill (a bill that Lib Dem MPs had earlier supported on its first trundle through Parliament). First, the NHS is uniquely precious to British voters, Clegg allies note: no other institution can match its status as a totem of state benevolence. Second—despite Mr Cameron's efforts to present himself as a defender of the NHS—the Conservative Party as a whole remains “massively vulnerable” to suggestions that they are planning to privatise the health service. Third, the Lib Dems were hammered on May 5th in local and regional elections, as well as in a referendum on whether to change Britain's electoral system. Private polling after that drubbing revealed that disgruntled Lib Dem supporters were unimpressed by talk of Mr Clegg securing policy wins within the coalition. Angry ex-Lib Dems wanted redder meat: proof that Mr Clegg was stopping wicked Tories from doing wicked things.

Put those three factors together, and a row about health was “exactly the right issue” for a party looking to demonstrate its new doctrine of “muscular liberalism”, says an ally of the deputy prime minister. In private, Mr Clegg still supports reform of the NHS, including greater use of non-state providers (though he thinks the political presentation of the reforms by the Tory health secretary, Andrew Lansley, has been wretched, and that the changes could have been achieved without new legislation). But given a chance to craft a political narrative about nice Lib Dems saving the NHS from nasty Tories, admits the Clegg ally, “we're going to get our pound of flesh.”

What does all this reveal about Mr Cameron's coalition? Were Conservative modernisers right to argue that two-party government makes difficult reforms easier to pull off? Or has the NHS row confirmed a deeper Tory hunch: that just as leopards have spots, Lib Dems are perfidious and sneaky, making the coalition an obstacle to bold policymaking? There is something to the first argument and, alas, the second too.

On the positive side of the ledger, the coalition set important reforms in motion in its first year. The two parties agreed to eliminate the budget deficit within one parliament, to give state schools much more autonomy and to redesign the welfare system so that taking a job almost always pays. Both Mr Cameron and Mr Clegg remain publicly committed to those reforms: many Lib Dems disagree, so Mr Clegg deserves credit for his consistency. Attacking the Tories on health offered a unique political prize, says a Lib Dem source, but his party is not going to make a habit of it: “you can't be a party of opposition in government.”

A Conservative minister is warier. The NHS row is a big event but an unreliable guide to coalition dynamics, he says. Nothing else is as sensitive as health policy, and the Lib Dems have been supportive elsewhere. Yet, he adds, it will have big consequences if the Lib Dems develop a taste for playing the “nice” party in the coalition.

Violence begets violence

The NHS row already matters. In the government's heady first months, says a Lib Dem source, it seemed as if an ideological synergy between Mr Clegg and Mr Cameron—and those around them—generated radical policies almost “automatically”. From now on, he admits, the coalition will be more “businesslike”, and each party leader will need to commit serious political capital if he wants backing for fresh reforms dear to his camp.

Even that may be too sunny a reading of the past few weeks. After May's bruising elections and referendum (during which, to be fair, Conservative Party proxies launched personal attacks on Mr Clegg), the Lib Dems have now stoked public suspicions that Mr Cameron cannot be trusted on the NHS.

There is a lot that still needs fixing in Britain, including the NHS. This coalition in-fighting must stop. By instinct, many Lib Dems are drawn to obstructionism atop the moral high ground. They should resist. By instinct, many Tories are overly suspicious of alliances—and have gained more than they care to admit from coalition rule. Coalition blood has been drawn, and trust damaged. Luckily, in politics self-interest is a pretty good substitute for trust. Britain's two governing parties have a vital interest in making their partnership work.