Directly changing Max GDP and Population using policies.

I want to make the parents voter group affect population growth and decline, what variable should I use to increase population?

Basically, I want the population to increase over time, is there a way to do this?

There is no way in the game to actually increase population of virtual voters, but if you want to simulate it’s effects, take a look at the immigration value, which puts pressure on other variables.
of course there is nothing to prevent you modding in an entirely new ‘population’ simulation value, affecting that, and using that as an input into other things…