If I knew Spending was the solution to economic problems, and worked every time, there would be no debate, no need to question. We both know there's no magic bullet and our quandary leaves us scratching for answers.
What I believe is that you can demonstrate how it works, temporarally. Yet other factors come into play which must be addressed or mitigated. Perhaps deficits are derived from those, and Stimulus is the one good answer for downturns. It could be Conservatives are chasing the wrong dragon here, but if that's the case we need to demonstrate it. Those other factors, which harm the economy, need to be listed and quantified for simple folks.
There's a way to turn around the opposition you face, but it's going to require a great deal of understanding of the economy and simple translation for us to follow. Think of it as a public information / education campaign.
Our opposition begins by simply thinking you've got to pull back and not let deficits get out of control. You can demonstrate that borrowed money is good for the economy, but how do you manage this borrowing in the long run?