Nate Silver has studied the effects of how immigration reform allowing undocumented immigrants to become US citizens may influence future presidential elections. He explains that the introduction of these additional citizens has much less effect than demographic changes already anticipated among existing citizens.
The article linked below includes an interactive graphic that allows readers to alter inputs to test how various changes might affect future presidential election outcomes. The graphic is preset to display reasonable assumptions.
New York Times
FiveThirtyEight: Nate Silver's Political Calculus