CREDIBILITY.

It is the all-important issue when it comes to managing the economy.

In the last parliament, Labour allowed itself to be painted for too long as the party which had "crashed the economy" and could not be trusted again with the car keys.

Poll after poll showed that when it came to economic competence, the voters trusted David Cameron and George Osborne to be at the wheel over Ed Miliband and Ed Balls.

And now with lots of economic numbers turning rosy, the Labour leader is desperate to place economic credibility along with social justice at the top of his agenda; just, incidentally, like Tony Blair did all those years ago.

So, we have the "budget responsibility lock"; a mechanism meant to ensure fiscal prudence and reassure the public that when it comes to managing the nation's money Labour would not be profligate.

"Every promise we make is a promise we can pay for," boasted Mr Miliband, attacking the Tories for £20bn of unfunded commitments, declaring it a sign of their "desperation and panic".

Labour's key economic policy is to eliminate the current budget deficit "as soon as possible in the next parliament" together with cutting the deficit and seeing overall debt fall as a percentage of GDP.

But as the Institute of Fiscal Studies pointed out - somewhat awkwardly for Labour on its big day - this phrase shrouded the Opposition's prime financial pledge in thick fog. When precisely would the day to day spending books be balanced?

Referring to the "big unknown", Paul Johnson, the IFS Director, said depending on the pace towards balancing the books Labour adopted, it could avoid £18bn of cuts over the next three years altogether.

"It allows them to say: 'Well, we would be cutting very little but also that we would be cutting.' But it really makes a big difference; there's a huge difference between £18bn of cuts over the next three years and no cuts.

"Literally, we would not know what we were voting for if we were going to vote for Labour," added Mr Johnson; rather unhelpfully for Labour HQ.

Of course, Jim Murphy, the Scottish Labour leader, pointed out how his party might not have to make cuts at all; only for the two Eds to insist, as part of their fiscal credibility drive, that, of course, there would be cuts ; only they could not say where exactly the axe would fall.

Mr Cameron denounced Labour's bid for fiscal credibility as "not a conversion to responsibility, it is a con trick"; more borrowing would mean more tax rises.

His Lib Dem deputy Nick Clegg sparked some controversy by likening Labour's bid for fiscal credibility to an alcoholic with a bottle-a-day habit, saying they had given up when no one believed them.

But just as there is a question-mark over where the Tories would find billions of pounds in unfunded commitments, there is another over how and when Labour would balance the books on day-to-day spending.

Both parties' messages seem to be: trust us, we'll manage it; everything will be fine.

Only on May 8 will we know who has passed the credibility test.