In this case, Professor Krugman is right for the wrong reasons. First of all, the markets are thirsty for Treasury Bills (recall that the government runs a deficit by selling Treasury Bills, Notes, or Bonds) -- if you doubt me, just look at how low are their yields and how high are their prices. If the Treasury has a product that the market likes, than it should supply it until those prices and yields indicate that the markets have had enough. [This is Milton Friedman's Optimum Quantity of Money, applied to interest-bearing government liabilities. See also an AER paper by Professor Woodford called "Public Debt as Private Liquidity"]. I don't know whether liquidity will stimulate the economy, but even if it didn't, it's still efficient to provide liquidity when its cost of production is less than the market's valuation of it.
Second, it's time to cut taxes. Actually, it's always time to cut taxes! Cutting spending is politically more difficult. So you cut taxes first and let the growing interest payments on the public debt be your partner in the crusade against public spending [Reagan ran this program well].
Professor Krugman's argument (New Keynesian, and by no means novel) for running a deficit is to stimulate the economy with higher spending and/or lower taxes. This argument is either wrong, or absurd, or both. In the interests of brevity, I will deal with the absurd side of it and discuss the rest later. The New Keynesian argument why deficits would stimulate the economy is that the economy is perpetually depressed by excessive monopoly, especially during recessions (the so-called counter-cyclical markups), and that fiscal (or monetary) stimulus is a roundabout way of forcing monopolists to reduce their prices. But that's the Justice Department's (aka, DOJ) job, and the Justice Department has tools that are much better suited for dealing with monopolists' anti-competitive practices. I repeat my Query to New Keynesians -- What do You Have Against the DOJ?.