The main contributor to over population in the United States since the 1960's has been immigration. Most significantly illegal immigration, false citizenship for children born to illegal immigrants, and overly generous secondary legal immigration (immigration of relations).
By 1965 the US had achieved zero population growth at 150 million people. Zero population growth scared the easy money made from population growth and over spending by governments dependant on ever increasing taxpayer population. Hence, the 1965 immigration law expanded immigration, and recent administrations have refused to enforce the limitations in that expansive law.
Besides the issue of giving away this country to those who will not even take the trouble to apply and obey our laws, illegal immigration and amnesty are the population explosion which is the core cause of most of our domestic problems (energy, water, pollution, congestion, ...).
We do not need cheap labor - our economy will adjust as necessary to achieve any desired output. Most illegal immigrants (about 25% in California construction) hold well paying jobs that are thus denied to Americans who need a good-paying job or a step-up job to build experience. These are not jobs Americans will not do - these are jobs Americans cannot get! Only about 3 percent of the illegal immigrants in California do agricultural labor. A newer problem is that automation is rapidly reducing the need for unskilled workers.
Estimates of our illegal population vary from 15 million to more than 30 million, with Mexicans accounting for 60 percent and Latin and Central Americans 24 percent. More than 10 percent of all Mexican citizens live in the US illegally! Remittances (money sent back to Mexico) are in the tens of billions each year, and is Mexico's second largest economic sector. Most of these illegal immigrants are uneducated persons accommodated to corrupt government and subservience, and put a burden on our social services. Educated and successful Mexicans stay in Mexico. Remember that every working immigrant means an American citizen on unemployment or welfare!
It matters not at all that these illegal immigrants are nice people just trying to have a life. I have been to many places in the world, and the whole planet is chock full of nice people. But they cannot all move here! There is no ethical problem, no one has the right to migrate to another's living space, no matter their problems at home.
The solution is enforcement of immigration laws against both illegal immigrants and employers, and detention and deportation. Also denial of the false citizenship granted to children of illegal immigrants born here (they are citizens of the their home country and not subject to our jurisdiction). Enforce existing law, deny all benefits, build the border fence to facilitate and clarify the border, investigate all SSN discrepancies, hold employers accountable including individuals, etc.