Last week, the two main parties set about each other on the question of future spending and borrowing. As Peter Hoskin explored yesterday, there will be a huge amount of forensic poring over, and furious argument about, exactly what Labour and the Conservatives are promising.

There’s always a danger in such territory – to go on the attack with impunity, you must feel your own pledges are impervious to assault. For the Conservatives, that confidence is leant by the polling which shows they far outstrip Labour in terms of trust – they wager that an offensive by Ed Balls will be trusted less by the electorate than their statements of defence.

If you lack credibility, as Labour do, then it’s always tempting to borrow someone else’s. That is precisely what they have done, looking to the OBR and the IFS for predictions of doom and disaster. Unfortunately for them, that sword cuts both ways – having endorsed such outside sources, they will now find it more difficult simply to brush off awkward statements from them about Labour’s plans.

As The Times reports today, the IFS have delivered just such an awkward statement:

‘Labour’s plan to soften spending cuts if it wins the election would mean borrowing up to £50 billion a year more by 2020 than the Conservatives, the head of Britain’s leading economic think-tank is warning.

Continuing to follow the path set out by Ed Miliband risks adding £170 billion to the national debt by 2030, leaving little room for the government to offer emergency help if there is another crisis, according to Paul Johnson, director of the Institute for Fiscal Studies.’

That’s gotta hurt. Any party would find such headlines uncomfortable, but for an outfit widely viewed as being incapable with money it’s a heavy blow.

Labour aren’t helped by the weakness of their lines and the incompetence of those they put up to deliver them. Asked to explain how the black hole could be filled, Chris Leslie told the Daily Politics a Miliband government would raise the price of gun licences.

Not only would that raise only £17 million, according to the Opposition’s own figures, but characteristically Yvette Cooper has already pledged to spend that money on the police, not on keeping down the deficit.