There are so many studies showing that it is definitely not true that
all pre-1936 economists were advocates of laissez-faire that it is
hard to know where to start listing them.

On the other hand, whether or not Krugman is right depends critically
on what you mean by "dominated" - what that word means here is just as
problematic as "free market orthodoxy".

I am not aware of any statistical analysis (which would be one way to
do this) because the problem has so many dimensions - micro and macro
for a start - and is riddled with difficulties. One starting point
would be the literature on demand management before Keynes, which shows
that interventionist policies were widely accepted in many quarters
before 1936.

It does not answer the question, but Michael Bernstein's book on the
US economics profession might be worth a look.

Roger Backhouse