If you’re a serious Keynesian, you’re for maintaining and even increasing spending when the economy is depressed, even though revenue has plunged; but you’re for fiscal restraint when the economy is booming, even though revenue has increased.
In other words, you want to (roughly) balance the budget over the course of an economic cycle, running a surplus during the expansionary phase and a deficit during the recessionary phase.
Obviously there are technical disputes about whether this is the right way to manage the macroeconomy. But in a way, it hardly matters. In the same way that real Christianity is simply too hard for mere mortals to practice (thus the need for a merciful God), so is hard Keynesianism. In the real world, you’re just never going to be able to persuade people to demonstrate the fiscal restraint that Keynesianism requires during boom times. And if, in the real world, Keynesianism is too difficult for human beings to practice during the 80% of the economic cycle taken up by expansion, then it’s not much good. Economics, after all, doesn’t feature a notably merciful God.
I don’t know if anyone has ever proposed a feasible way to overcome this problem.