Funding record borrowing
What I'll be looking out for in today's Budget are three things.
First, what the Treasury and the Debt Management Office have to say about how they intend to borrow the record amounts that the government needs to raise from investors.
Second, how the government bond or gilts market responds to new hair-raising disclosures on debt that has to be financed - and the impact on the value of the pound.
Third, what the Treasury factors in for growth in the economy after we're through the recession - because the credibility of plans to pay back all this extra borrowing will hinge to a large extent on whether its projections for the recovery are perceived to be excessively optimistic.
Here are the basic numbers that, I think, will frame today's debate.
In the last Budget, a year ago, Alistair Darling projected public-sector net borrowing for 2009/10 of £38bn.
Come the autumn and his pre-Budget report, he projected that net borrowing for that period would be more than three times greater, at £118bn.
And today, just a few months on, he's expected to say he's understated yet again the scale of what the public sector needs to borrow - and that public sector net borrowing in the current fiscal year will be nearer to £180bn.
That would be an unprecedented amount for peacetime, both in absolute terms (natch) and as a percentage of GDP (more than 12%).
Now there are two ways it can raise this money.
The conventional way is to borrow it from investors, such as pension funds, insurance companies, hedge funds and other central banks.
It does this by selling gilts or government bonds to them - or rather the Debt Management Office, an offshoot of the Treasury, does this on behalf of the government.
As of now, the Debt Management Office expects to sell £148bn of new gilts into the market this year.
But that is bound to be revised up later today - to nearer £200bn.
And the closer that new aggregate amount for gilt sales is to £200bn, the more likely it is that investors will be spooked - since most analysts are expecting gilts sales closer to £180bn.
It would therefore help the government's cause if it could come up with a wheeze to sell this debt in forms that are more attractive to investors. For example, pension funds have for years been urging the government to issue debt in the form that more neatly matches their long term liabilities.
We've also, of course, all been recently reminded that there is a less conventional way of funding this deficit - which some would describe as dangerous and inflationary.
That is to turn public sector debt into money (to monetize it) through purchases of gilts by the Bank of England.
To ease putative deflationary pressures, the Bank of England is doing just that through its quantitative easing programme.
However Paul Fisher, the Bank of England's markets director, yesterday said the Bank's current budget for buying gilts and other forms of debt, which is £75bn, looked "about the right size" and was "calibrated reasonably well".
The price of gilts fell in response, because of investors' disappointment that demand for gilts from the Bank wouldn't be higher.
It would be odd if the gilt market weren't nervy today.