Brad DeLong, speaking of Paul Krugman, quotes the preface to Keynes's Essays in Persuasion. Keynes called the essays the "The croakings of a Cassandra who could never influence the course of events in time." Brad revises "I would not, however, say "never"--I would say "sometimes, and we all hope more in the future than in the past"."
The point that you slowly improve the state of society is an important one, and one that I think is often ignored by people who say "if politicians will never do exactly what you're happy with, then you have to abandon the whole Keynesian project as unworkable". First, that doesn't make any sense because Keynesianism is an approach to macroeconomic science. If it's right, it's right regardless of whether politicians pay attention to its rightness. This dependence on what politicians do is a non-sequitor.
But even if we think about policy suggestions that may emerge from Keynesian macroeconomics, it still doesn't make sense to abandon the whole thing just because of resistance. As Brad suggests, we can still make progress. Although there was only a weak, early Keynesian response to this crisis it was still better than the catastrophe of the 1930s which took several years for an adequate monetary response and even longer for a substantial fiscal response. Some people talk as if it should be obvious that a policy we think would have a negative impact should be prefered to an inadequately implemented policy that we think would have a positive impact. This makes no sense at all, but you see the argument all over the place as if it's some deep wisdom.
We could turn the tables as a demonstration here. Politicians - even ones that talk sorta like true blue libertarians - have not embraced "true" libertarianism, and they aren't going to embrace it any time soon. So should libertarians just drop their views because they are unworkable in the real world? They apparently don't think so. So why is this considered such an ace-in-the-hole argument against a timid, un-followed-through-on Keynesianism?
I like Keynes at the end of that preface too (the whole book is very good):
"I have thought it convenient to choose this date of publication, because we are standing at a point of transition. It is called a National Crisis. But that is not correct - for Great Britain the main crisis is over. There is a lull in our affairs. We are, in the autumn of 1931, resting ourselves in a quiet pool between two waterfalls. The main point is that we have regained our freedom of choice. Scarcely any one in England now believes in the Treaty of Versailles or in the pre-war Gold Standard, or in the Policy of Deflation. These battles have been won - mainly by the irresistable pressure of events and only secondarily by the slow undermining of old prejudices. But most of us have, as yet, only a vague idea of what we are going to do next, of how we are going to use our regained freedom of choice."