ON PAPER, Ed Miliband, the Labour leader, and his shadow chancellor Ed Balls, had a pretty successful day today. It was standing room only at a Monday morning press conference the pair held at Church House, a short walk from Parliament: in itself not a bad achievement for an opposition party that could be four years away from the next general election. What is more, the Westminster commentariat spent the rest of the day debating the credibility of the main proposal unveiled by the two Eds, namely that if a special tax on bank bonuses (which raised billions of pounds when tried as a one-off super tax by Labour) were repeated this year, it could create 110,000 new jobs.

From the left, outlets like the Daily Mirror reported this alternative Labour plan for growth in neutral terms, saying:

Mr Miliband said that the Government would raise an estimated £2 billion from repeating the one-off windfall tax on bank bonuses imposed by former chancellor Alistair Darling.

He told a Westminster news conference that the revenue could be used to build 25,000 new homes at a cost of £1.2 billion, generating 20,000 additional jobs in the construction industry.

A further £600 million could be used to create a fund for youth jobs, helping 90,000 into work and apprenticeships, while Labour would use £200 million to boost the regional growth fund

From the right, the Spectator wondered if a second attempt at a bankers' bonus tax would really have raised so much money, citing a comment by one Gordon Brown, to the effect that banks had learned how to avoid paying the tax.

Over at PoliticsHome, Paul Waugh shrewdly noted how Mr Balls (a dyed-in-the-wool Keynesian), seemed to be hinting that if he were chancellor in a Labour government today he might be rowing back from a plan by the last actual Labour chancellor, Alistair Darling, to halve the deficit over four years.

The Financial Times wondered if another part of the two Eds' proposal, to reverse a rise in VAT levied on fuel, might break European Union rules.

On paper, then, a successful bid to set the news agenda for the Westminster village, just over a week before Mr Osborne delivers the 2011 Budget. Mr Balls and Mr Miliband got out their attack lines about how extreme Mr Osborne's spending cuts are. Mr Balls managed to remember to sound humble, saying: "Of course we didn't spend every penny wisely", while still trotting out his favourite defensive lines about how Britain, under Labour, had gone into the credit crunch in better financial shape than lots of other big countries.

The pugnacious Mr Balls even remembered to allow his boss and former leadership rival, the rather meeker Mr Miliband, to answer questions first and generally take the lead, glowering attentively (if not quite admiringly) while the meeker Ed spoke.

But sitting in the room at the press conference, a simpler thought occurred to me. Given a rare chance at grabbing the limelight in opposition, Labour's new leaders had chosen to have us debate Labour proposals. More specifically, Mr Miliband, the new and not very secure Labour leader, had allowed his big press conference for this month to become a chance for Mr Balls to explain, with numbers, why he would be a better chancellor of the exchequer, right now, than Mr Osborne (and indeed hint that he would have been a wiser chancellor than Mr Darling).

What were they thinking?

Some 12 months after the last election, the job of the opposition is not to make voters regret their foolishness in rejecting the Labour party in 2010. The job of the opposition this early in a parliament, surely, is to tear chunks out of the sitting government, and not much else. The job of Mr Miliband is to sound vaguely prime ministerial, while hinting that Mr Cameron has no credibility, no heart, no shame or what have you. The job of Mr Balls, a clever and caustic man who knows his economics, should be to rip into Mr Osborne's competence and attempt to spike the guns of his Budget with the aid of a brilliant, simple-but-telling soundbite defining the Tory's plans before Mr Osborne has the chance to define them.

Instead, Mr Balls indulged what appears to be a boundless desire to justify his own record in office and to set out all the marvellous things he would be doing if only he were in charge right now, complete with thickets of statistics and cheering forecasts of spurious precision, in the grand Gordonian manner. The official Labour Party news release about the press conference is almost wholly devoted to lauding the Balls alternative plans, and carries the sub-heading:

£2billion bank bonus tax to create 110,000 new jobs

Memo to Mr Balls: you are not running the country any more.

More worryingly still for the Labour party, Mr Miliband indulged Mr Balls in this.