In a recollection of his involvement the early days of supply-side economics, Bruce Bartlett in the New York Times characterizes the mainstream view the supply-siders were up against as follows:

It’s important to remember that at the time supply-side economics came into being, Keynesian economics dominated macroeconomic thinking and economic policy in Washington. Among the beliefs held by the Keynesians of that era were these: budget deficits stimulate economic growth; the means by which the government raises revenue is essentially irrelevant economically; government spending and tax cuts affect the economy in exactly the same way through their impact on aggregate spending; personal savings is bad for economic growth; monetary policy is impotent; and inflation is caused by low unemployment, among other things.

In a comment on Mark Thoma’s blog, Paul Krugman claims that Bartlett’s Keynesianism is a straw man:

Wow. You see, I was a grad student at MIT - the great Keynesian stronghold - in the 1970s, and this bears no resemblance to what was being taught.

Really? Not even to undergraduates? Just a few blocks up Massachusetts Avenue, I was a freshman taking Economics 10 at Harvard in 1973-74. The course was led by Otto Eckstein; my section was taught by Chip Case; our textbook was Lipsey and Steiner. Based on my experience, I find Bartlett's characterization of the Keynesian macroeconomics of time quite accurate.

"Budget deficits stimulate economic growth": We didn’t hear much about growth, but we were certainly taught that deficits raise the level of income, via the balanced-budget multiplier.

"The means by which the government raises revenue is essentially irrelevant economically": Well, in the macro half of the course, yes.

"Government spending and tax cuts affect the economy in exactly the same way through their impact on aggregate spending": Yes.

"Personal savings is bad for economic growth": Certainly bad for the level of income, due to the paradox of thrift.

"Monetary policy is impotent; and inflation is caused by low unemployment, among other things." Yep. We were never taught MV=Py. We actually did hear that the bad anchovy harvest was a cause of inflation. I recall that an Ec 10 teaching assistant named Roger Brinner wrote an article about the causes of inflation for the Harvard Independent (an undergrad newspaper) in which he made not one mention of the word "money", much less any reference to growth in the supply of money. I remember it well because I wrote a reply for the Independent which became my first published writing on economics.