In the simplest case, people want to hold a stock of money in some proportion to their total income. Money is produced only by the government. Now suppose people's demand for money rises, and the government fails to increase supply accordingly. You might expect the price of money to rise -- that is, deflation. But deflation doesn't restore equilibrium, either because prices are sticky (i.e., deflation can't happen, or not fast enough), or because deflation itself further raises the demand for money. It might do this by raising precautionary demand, since falling prices make it likely that businesses and households won't be able to meet obligations fixed in money terms and will face bankruptcy (Irving Fisher's debt-deflation cycle). Or deflation might increase demand for money by because it redistributes income from net borrowers to net savers, and the latter have a higher marginal demand for money holdings. Or there could be other reasons. In any case, the price of money doesn't adjust, so government has to keep its quantity growing at the appropriate rate instead. From this perspective, if we ever see an economy operating bellow full capacity, it is true by definition that there is excess demand for some money-like asset.
This sounds like Milton Friedman. It is Milton Friedman! But it also seems to be most of the liberal macroeconomists who are usually called Keynesians. Here's DeLong:
there was indeed a "general glut" of newly-produced commodities for sale and of workers to hire. But it was also the case that the excess supply of goods, services, and labor was balanced by an excess demand elsewhere in the economy. The excess demand was an excess demand not for any newly-produced commodity, but instead an excess demand for financial assets, for "money"...That's an admirably clear statement. But is it right? I mean, first, is it right that demand constraints can always and only be usefully characterized as excess demand for some financial asset? And second, is that really what the General Theory says?
How, exactly, should economists characterize the excess demand in financial markets? Where was it, exactly? That became a subject of running dispute, and the dispute has been running for more than 150 years, with different economists placing the cause of the "general glut" that was excess supply of newly-produced goods and of labor at the door of different parts of the financial system.
The contestants are:
Fisher-Friedman: monetarism: a depression is the result of an excess demand for money--for those liquid assets generally accepted as means of payment that people hold in their portfolios to grease their market transactions. You fix a depression by having the central bank boost the money stock...
Wicksell-Keynes (Keynes of the Treatise on Money, that is): a depression happens when there is an excess demand for bonds... You fix a depression by either reducing the market rate of interest (via expansionary monetary policy) or raising the natural rate of interest (via expansionary fiscal policy) in order to bring them back into equality....
Bagehot-Minsky-Kindleberger: a depression happens because of a panic and a flight to quality, as everybody tries to sell their risky assets and cuts back on their spending in order to try to shift their portfolio in the direction of safe, high-quality assets... You fix a depression by restoring market confidence and so shrinking demand for AAA assets and by increasing the supply of AAA assets....
From the perspective of this Malthus-Say-Mill framework Keynes's General Theory is a not entirely consistent mixture of (1), (2), and (3)...Note that these financial market excess demands can have any of a wide variety of causes: episodes of irrational panic, the restoration of realistic expectations after a period of irrational exuberance, bad news about future profits and technology, bad news about the solvency of government or of private corporations, bad government policy that inappropriately shrinks asset stocks, et cetera. Nevertheless, in this Malthus-Say-Mill framework it seems as if there is always or almost always something that the government can do to affect asset supplies and demands that promises a welfare improvement
The first answer is No. Or rather, it's true but misleading. It is hard to talk sensibly about a "general glut" of currently produced goods except in terms of an excess demand for some money-like financial asset. But recessions and depressions are not mainly characterized by a glut of currently produced goods. They are characterized by an excess of productive capacity. Markets for all currently-produced goods may clear. But there is still a demand constraint, in the sense that if desired expenditure were higher, aggregate output would be higher. The simple Keynesian cross we teach in the second week of undergrad macro is a model of just such an economy, which makes sense without money or any other financial asset. (And is probably more useful than most of what gets taught in graduate courses.) Arguably, this is the normal state of modern capitalist economies.
I'll come back to this in a future post, hopefully. But it's important to stress that the notion of aggregate demand limiting output, does not imply that any currently-produced good is in excess supply. 
Meanwhile, how about the second question -- in the General Theory, did Keynes see demand constraints as being fundamentally about excess demand for money or some other financial asset, with the solution being to change the relative price of currently produced goods, and that asset? Again, the answer is No.
In his explanation of the instability of capitalist economies, Keynes always emphasizes the fluctuations in investment demand (or in his terms, the marginal efficiency of capital schedule). Investment demand is based on the expected returns of new capital goods over their lifetime. But the distribution of future states of the world relevant to those returns is not just stochastic but fundamentally unknown, so expectations about profits on long-lived fixed capital are essentially conventional and unanchored. It is these fluctuations in expectations, and not the demand for financial assets as expressed in liquidity preference, that drives booms and slumps. Keynes:
The fact that a collapse in the marginal efficiency of capital tends to be associated with a rise in the rate of interest may seriously aggravate the decline in investment. But the essence of the situation is to be found, nevertheless, in the collapse of the marginal efficiency of capital... Liquidity preference, except those manifestations which are associated with increasing trade and speculation, does not increase until after the collapse in the marginal efficiency of capital.
It is this, indeed, which renders the slump so intractable. Later on, a decline in the rate of interest will be a great aid to recovery and probably a necessary condition of it. But for the moment, the collapse in the marginal efficiency of capital may be so complete that no practicable reduction in the rate of interest will be enough [to offset it]. If the reduction in the rate of interest was capable of proving an effective remedy by itself, it might be possible to achieve a recovery without the elapse of any considerable interval of time and by means more or less directly under the control of the monetary authority. But in fact, this is not usually the case.In this sense, Keynes agrees with the Real Business Cycle theorists that the cause of a decline in output is not fundamentally located in the financial system, but a fall in the expected profitability of new investment. The difference is that RBC thinks a decline in expected profitability must be due to genuine new information about the true value of future profits. Keynes on the other hand thinks there is no true expected value in that sense, and that our belief about the future are basically irrational. ("Enterprise only pretends to itself to be actuated by the statements in its prospectus ... only a little more than an expedition to the South Pole, is it based on an exact calculation of benefits to come.") This is an important difference. But the key point here is the bolded sentences. Keynes considers DeLong's view that the fundamental cause of a downturn is an autonomous increase in demand for safe or liquid assets, and explicitly rejects it.
The other thing to recognize is that Keynes never mentions the zero lower bound. He describes the liquidity trap as theoretical floor of the interest rate, which is above zero, but nothing in his argument depends on it. Rather, he says,
The most stable, and least easily shifted, element in our contemporary economy has been hitherto, and may prove to be in the future, the minimum rate of interest acceptable to the generality of wealthowners. (Cf. the nineteenth-century saying quoted by Bagehot, that "John Bull can stand many things, but he cannot stand 2 percent.") If a tolerable level of employment requires a rate of interest much below the average rates which ruled in the nineteenth century, it is most doubtful whether it can be achieved merely by manipulating the quantity of money.This is an important part of the argument, but it tends to get ignored by mainstream Keynesians, who assume that monetary authority can reliably set "the" interest rate. But as we see clearly today, this is not a good assumption to make. Well before the policy rate reached zero, it had become effectively disconnected from the rates facing business borrowers. And of course the hurdle rate from the point of view of the decisionmakers at a firm considering new investment isn't just the market interest rate, but that rate plus some additional premium reflecting what Keynes (and later Minsky) calls borrower's risk.
So, Keynes thought that investment demand was subject to wide, unpredictable fluctuations, and probably also a secular downward trend. He doubted that very large movements in the interest rate could be achieved by monetary policy. And he didn't think that the moderate movements that could be achieved, would have much effect on investment.  Where did that leave him? "Somewhat skeptical of the success of a merely monetary policy directed toward influencing the rate of interest" at stabilizing output and employment; instead, the government must "take an ever greater responsibility for directly organizing investment."
Of course, DeLong could be misrepresenting Keynes and still be right about economic reality. But we need to at least recognize that aggregate demand is logically separate from the idea of a general glut; that the former, unlike the latter, does not necessarily involve excess demand for any financial asset; and that in practice supply and demand conditions in financial markets are not always the most important or reliable influences on aggregate demand. Keynes, at least, didn't think so. And he was a smart guy.
 The other point, to anticipate a possible objection, is that the investment decision does not involve allocation of a fixed stock of savings between capital goods and financial assets.
 The undoubted effectiveness of monetary policy in the postwar decades might seem to argue against this point. But it's important to recognize -- though Keynes himself didn't anticipate this -- that in practice monetary policy has operated largely though its effect on the housing market, not on investment.