From “the best-selling Principles of Economics textbook [which] has been teaching students in a clear, unbiased way for 40 years.” Campbell R. McConnell, Economics: Principles, Problems, and Policies.  McGraw-Hill, 7th edition, 1978.

The Last Word: The Impotence of Fiscal Policy
Some economists feel that fiscal policy is an impotent and unpredictable stabilization tool.

Well, as I wrote in a 1977 Tax Review article (reprinted 1w permission of the Tax Foundation, Inc.):

The real question is whether or not conventional fiscal policy works as advertised. If fiscal policy works, and its impact is properly measured by the size of the full employment deficit, then it should be possible to find some correlation between either the level or direction of the full employment budget and some measure of current or subsequent economic activity. George Terborgh tried to find some such link back in 1968, in The New Economics, but found only a weak correlation that turned out to be perverse. That is, larger full employment surpluses were associated with faster economic growth. More rigorous tests by economists at the St. Louis Fed, and again at Citibank, had no more luck in uncovering the magical properties of the full employment budget. A sharp shift toward larger full employment deficits did not prevent the recession of’ 1953-54, for example, nor the mini-recession of 1967. In 1946, a $60 billion reduction of Federal spending (equivalent to $400 billion today) was followed by a vigorous boom, and a combination of tax cuts and higher spending in 1948 (the equivalent of S75 billion today) was followed by a sharp recession.

The theory of fiscal policy is almost as messy as the evidence. If deficit spending is financed by borrowing from the private sector, there is no obvious stimulus-even to that undifferentiated thing called “demand.” Whoever buys the government securities surrenders exactly as much purchasing power as is received by the beneficiaries of Federal largess. There would be a net fiscal stimulus only if there were no private demand for the funds needed to cover the added Treasury borrowing. Otherwise, lendable funds are just diverted from market-determined uses to politically determined uses.

There may be a stimulus in some circumstances if the deficit is financed by a more rapid increase in the money supply, but this is really a monetary stimulus, not a purely fiscal effect.

In the long run, resources allocated through the government must displace those allocated through markets, and growth of government spending must be at the expense of the private sector. The government has only three sources of revenue – taxes, borrowing, and printing money – and increasing any one of those must reduce the private sector’s command over real resources. Although deficit spending may at times be a short-run stimulus to nominal demand, it is also a long-run drag on real supply-siphoning resources from uses that would otherwise augment the economy’s productive capacity, and instead diverting those resources into hand-to-mouth consumption through government salaries, subsidies, and transfer payments.

So, the theory and evidence suggests that fiscal policy is essentially impotent, or at least unpredictable, except as a device to promote inflationary monetary policy and/or to reduce investment and growth.