The problem is that in terms of economic policy, there's not much to unite on. The left is pushing modern monetary theory, neverending quantitative easing and stimulus, universal basic income, nonsense wealth taxes, etc. etc.
At this point, the fundamental ideological differences between left and right are simply too profound to overcome.
You either think capitalism is the best economic system and has allowed more people to lift themselves out of poverty than any other economic system in history..... or you're a Democrat and oppose that.