George Osborne gleefully unveiled analysis on Monday that suggested Labour would spend billions more if it won this year's election.
The Chancellor dismissed accusations from reporters that the "cost analysis of Labour party policy" by Treasury civil servants, working under assumptions about Labour policies made by Tory aides, was just a "load of nonsense".
Labour reacted with fury, saying it was a "dodgy dossier riddled with untruths" about their spending plans.
Shadow chancellor Ed Balls went on: “It isn’t an impartial exercise but a political smear based on false assumptions made by Tory advisers, including dozens of claims which are not even Labour’s policies."
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Osborne would understand his rival's irritation all too well, having lashed out at Labour ministers in 2010 when they produced their own dossier in government just before the general election attacking Tory spending plans.
David Cameron led the Tory response, calling the publication a "dodgy dossier full of lies" and "complete junk". Meanwhile, then shadow chancellor George Osborne said: "The credibility of Labour lies about Conservatives has collapsed."
In eerily similar tones to Balls' complaint, the senior Tory went on: "The dossier includes commitments we have never made, wild exaggerations of our costed policies, and, in some cases, admissions that some changes would actually be cheaper than we have budgeted for."
In fact, Osborne appears to have quite a talent for railing at something in opposition, before doing exactly the same in government.
In January 2010, he tore into the then Labour chancellor Alistair Darling's proposed "Financial Responsibility" bill, which sought to enshrine in law a Labour pledge to halve the deficit in two years. Fast forward nearly five years, and Osborne himself is now keen on pushing his own very similar bill (or "Charter of Budget Responsibility"), despite having once labelled such measures a "con".
You could say, with apologies to George Bush and John Kerry, that Osborne was against dodgy dossiers before he was for them.