Yet it is perfectly possible to make precisely the opposite case. Of the several grounds for doing so, one is so obvious that it is scarcely ever mentioned. The irony is that there is almost certainly a higher proportion of Tribune-friendly taxers and spenders in the Liberal Democrat party than there are in New Labour. It is conceivable that Paddy Ashdown's likeliest successor, with his emphasis on "social justice" - the very words which resound through the Tribune text - will turn out to be one of them. Yet this is the party to which the signatories react as Dracula to the cross.
True, those most unashamed of taxing and spending among the Liberal Democrats happen also to be those least keen on co-operation with New Labour. But, with the exception of Europe, which is left-right neutral, it is hard to think of a single criticism of the present Government by even the most coalitionable of the Liberal Democrats, which has not been from a relative position which could vulgarly be described as to the left. Even the liberal criticisms of non-economic policies like asylum or compromise on House of Lords reform or freedom of information are ones that would be shared, if not prioritised, by the old Labour left. If the tribalists' unspoken fear about closer links with the Liberal Democrats is that Blair would be less dependent on some of his MPs on the left, then let them say so. But to suggest that co-operation with the Liberal Democrats is a right- wing plot is an obsolete story that doesn't stand up.
But the argument goes further than that. Leave aside the historic oversimplification - to put it politely - that Labour was born out of the "failure" of "19th- century Liberalism" rather than from the trade unions' desire to form their own party. (An odd "failure" that, which produced, on the back of a Liberal landslide of Blair-like proportions, the radical Lloyd George Budget of 1909.) Consider instead proportional representation, the necessary condition of the sustained Lib-Lab co-operation which the Tribunites are so straining to prevent.
It is hardly surprising that one of the signatories outside the ranks of the left is the militantly anti-PR Stuart Bell. And yet the impact of PR, as its more prominent defenders on the left - such as Robin Cook - have recognised, has a quite opposite potential to that which its old Labour opponents purport to fear.
Under the first-past-the-post system it is much easier to ignore the disillusioned "core vote" than under PR. To put it in crude party management terms, low turnouts in Labour strongholds don't matter to election outcomes under the First Past the Post system. Low turnouts in Labour strongholds in elections carried out by proportional representation matter very much indeed, as the European election results graphically illustrated. If it's true, as some of the signatories to the Tribune statement would certainly claim, that there was a lower turnout in safe Labour seats because the Government is not socialist enough and too focussed on Middle England, then the way to frighten it into changing direction is by introducing PR.
Finally, there is a point about Lib-Lab co-operation, which has to do not with the direction of the Labour Party but with its electoral fortunes - a consideration to which the Tribune signatories are presumably not immune. It is too easily forgotten in both the Liberal Democrat and Labour parties how far they were both advanced by tactical voting in the 1997 election.
Professor John Curtice calculates that new tactical voting was decisive in around 15-21 of the seats which Labour gained then - more than in any previous election. He also points out that his own estimate is more conservative than some others. This may not sound like much in the context of a huge landslide; it is quite a lot in elections which are closer - the equivalent of a majority of 42 in fact. And it happened in 1997 because the parties barely attacked each other and had, as a matter of deliberate strategy, begun to lodge themselves in the electorate's mind as mutually co-operative. At the risk of ramming home the point, the same electorally beneficial co-operation is unlikely without a fairly bankable commitment to a PR referendum in the next Labour general election manifesto.
None of this is to say that there is no issue for the Labour leadership to address in this and other manifestations of unease in sections of the party. Predominantly, this is still a matter of message rather than substance. In a lecture last year, the MP Chris Mullin summarised some of the distinctly Labour achievements of the present Government: the minimum wage; the windfall tax and the New Deal for the unemployed; the redistributionist raid on the pension funds, the signing of the Social Chapter.
Yet these remain the curiously secret successes of the present Government. Partly this stems from an overwhelming desire not to disturb the consensus of which the Daily Mail is assumed to be the fulcrum. This strategy may have to be modified as the next general election approaches. It will be just as important to re-engage the activists on which victory will depend much more than it did in 1997. But there has also been a too frequent failure of will by the Cabinet as a whole to articulate a sense of collective purpose for this actually rather radical government.
Ministers are, of course, caught in a dilemma; some of the well-justified accusations of managerialism and "departmentalitis" miss the point that delivery, rather than electioneering, is now the vital imperative. Just how imperative is randomly illustrated by the recent opinion poll finding that the electorate believes that the NHS has got worse since the general election. But that does not absolve the ministeriat from being political, from engaging with the party on why it is acting as it is.
There are tough as well as tender messages lurking here: tender, in that the party needs to be reminded a little less obliquely how much the Government has done what the Tories wouldn't have. Tough, in that the left need to be reminded that tax and spend will only ever attract the voters, including "core" voters, if and when they believe that their money is being well spent. What they shouldn't worry too much about is the peddling of the myth that co-operation with the Liberal Democrats is somehow a betrayal, in the widest sense, of the left.