Confusion around immigration law has caused many Americans to support unrealistic immigration solutions. U.S. immigration law is set by statute passed by the legislative branch. Some parts of immigration law have been shaped by the courts as well. The president is responsible for implementation, which requires a number of decisions that are not necessarily spelled out in the legislation.
The federal laws regulating the deportation of an immigrant can be found here in section 304 of the U.S. Code. An alien must be provided at least ten days advance notice of a deportation hearing, which must be held in front of an immigration judge. At the hearing the alien is expected to provide proof of legal entrance or demonstrate evidence of belonging to an exempted group. There are several statutes that allow for an alien to remain in the United State despite illegal entry. If the judge rules the alien is here unlawfully and does not meet the requirements for an exception, the law allows the alien thirty days to file an appeal.
Imagine for a moment that the decision was made to mass-deport all illegal aliens living in the United States. According to the Department of Homeland Security there are 11.5 million unauthorized immigrants living in the United States. Currently there are 227 immigration-specific judges working on 375,000 pending cases, currently each case takes approximately 3 1/2 years to work through the court system. Even if you were to assume that each initial court hearing took only fifteen minutes, it would take 2,875,000 hours (or 328 years) for one judge to hold a 15-minute hearing for each of the 11.5 million illegal immigrants. If a judge was to work full time without vacation holidays or breaks and spent all day listening to cases, he/she would be able to hear 8348 cases in the course of a year. To address the hypothetical case load, we would need 1378 full-time immigration judges in order to deport all immigrants within one year, a staff increase of 600%.
The law does not specify in which order the 375,000 backlogged cases need to proceed. The president has decided to prioritize some cases over others. Immigrants who have been convicted of other crimes and new arrivals are being prosecuted first, while those who were brought into the country as children and those related to American citizens are prosecuted last. Due to this prioritization, some people may never be prosecuted. The legal term used in the executive orders for these latter cases is deferred action, not amnesty. These immigrants have not been granted amnesty because they are still technically scheduled to go before an immigration judge, many of these people whose cases have been deferred are likely to qualify to remain in the United States. In order to allow these immigrants to provide for themselves until their court hearings the president has ordered the attorney general to grant them work authorization as the attorney general is authorized by statute(8 USC § 1324a(h)(3)) to do. However, others will point to 8 USC § 1226 a(3) as evidence that the president cannot actually legally grant work authorization. Ultimately, Congress passed two contradictory laws and either argument may be valid; these cases will have to be dealt with individually by the courts.
It is estimated that we spent $2 billion to incarcerate 400,000 unauthorized immigrants in 2010. If you extrapolate that to 11.5 million illegal immigrants, that would be $57.5 billion more per year that we would need to spend on federal prisons if we were to try and keep them all in prison until they had their day in court. Once the aliens have been processed through the courts, they need to be deported to their home country. It is commonly but mistakenly assumed that the vast majority of unauthorized immigrants are Mexican; in reality, it's less than sixty percent, or barely over half. Some 4.6 million illegals are from other countries, and can not just be "driven back to Mexico."
Obviously, in order to to mass-deport 11.5 million people Congress would have to dramatically increase the budgets and staff of enforcement officials, courts, and prosecutors. Realistically, it would take decades and cost billions.
Immigration law is created by acts of congress, implemented by the president and reviewed by the courts. While some solutions seem obvious to many, they are often cost-prohibitive, forcing the president to make decision about how to implement the law. Many refer to the presidents actions as 'lawlessness", but he has actually been working with in the boundaries of congressionally passed statutes. Mass deportation, as advocated by Republican candidates, would require a violation of budget statutes.