This section of DiscoverTheNetworks examines the major trends that have characterized illegal immigration to America over the past few decades. The leading cause of the spike in the illegal population during that period has been a massive influx of Mexicans entering the U.S. at an unprecedented rate. Consider that during the entire decade of the 1950s, only about 300,000 legal Mexican immigrants came into the United States, making up 12 percent of the immigrant flow.

Illegal immigration from Mexico spiked initially in the late 1960s, after the abrupt termination (in 1964) of the bracero guest-worker program that had brought almost 5 million Mexican-born farm laborers to the United States between 1942 and 1964. Though the program was beset by some abuses, it had been successful overall.

In the 1980s and 1990s, the flow of cross-border migration became tidal. It is estimated that as of late 2009 there were 12 to 20 million illegal aliens residing in the United States -- two-thirds of them being of Mexican origin.  According to the Center for Immigration Studies, this trend began to moderate  in mid- to late 2007. The Department of Homeland Security (DHS) estimates that whereas the illegal alien population grew by 3.3 million between January 2000 and January 2007 (and then peaked in the summer of 2007), it subsequently declined by 180,000 over the next twelve months. Altered economic conditions in the U.S. played a role in this decline. DHS also attributes the decline to increased enforcement measures during the latter years of the Bush administration. These measures included:

• the construction of a security fence along portions of the U.S.-Mexico border;
• a doubling of the number of Border Patrol agents;
• a dramitic increase in the number of local law-enforcement personnel participating in the 287(g) program, which allows local police to enforce immigration laws;
• the doubling of the E-Verify program, which allows employers to screen workers to determine if they are authorized to work in the United States;
• stepped-up worksite enforcement efforts that led to a five-fold increase in criminal and administrative arrests between 2005 and 2009; and
• state and local governments' increased acceptance of initiatives designed to help the federal government enforce immigration laws.

Fighting against the foregoing enforcement efforts is an aggressive immigrant-rights lobby -- spearheaded by groups like the Mexican American Legal Defense and Education Fund (MALDEF) -- whose top public-policy priority is to fight on behalf of the voting rights of illegal aliens.

These rights are already exercised in ways other than actually voting in elections. The apportionment of U.S. House of Representatives seats by the Census is based on each state's total population—including illegal aliens and other non-citizens—relative to the rest of the country. Almost seven million illegal aliens were counted in the 2000 U.S. Census. As a result, California, a left-leaning state in which one in seven residents was a non-citizen, gained six House seats, a fact that in itself had a significant effect on the country’s politics. New York, Texas, and Florida also each gained a seat due to non-citizen residents. Those nine seats came at the expense of Indiana, Michigan, Mississippi, Montana, Oklahoma, Pennsylvania, Wisconsin, Kentucky, and Utah. Since the electoral votes which determine the winner in national presidential elections are based on the number of House and Senate seats each state has, the use of illegal aliens and other non-citizens in apportionment can also affect who occupies the White House.