An understanding that population growth reduces freedom.
Once that is understood, then it's a matter of weighing the loss of freedom against the benefits of immigration--both near term and long term.
Another possiblity is that no one should be allowed to become an American citizen unless, in addition to the usual requirements, he is able to purchase the citizenship of a current American, who must then leave the country.
Now I get it. Much momentum to your effort?
So?
After a while, people inclined to abort their children will have selected their genes out in the course of evolution--such people, by aborting their pregnancies, will make their kind extinct.
Left alone to run its course, and your problem will solve itself.
In the materialist's world, yes, women would be genetically predisposed to abort their children.
In the real world, abortion bespeaks a soul sickness, as countless women who've aborted will testify.