To understand who has the power to change U.S. immigration law, you must first understand how U.S. immigration law gets made. Immigration is not mentioned in the U.S. Constitution or any of its amendments, which are the supreme law of the land.
However, the U.S. Supreme Court ruled early on that immigration regulation was an exclusive responsibility of the federal government. From time to time state and local (city or county) governments attempt to make laws that affect immigration, but they're usually unsuccessful.
There are three branches of federal government—the executive (headed by the president), the legislative (Congress), and the judiciary (the courts)—each of which has the power to make and change the rules of immigration.
Federal laws are written by Congress and become effective when signed by the U.S. president or when a presidential veto is overridden. The Immigration and Nationality Act, which is the main law governing immigration in the United States, came from Congress, and it is Congress that has the power to change it or to write any other laws affecting immigration.
The agencies of the executive branch of government are responsible for administering Congress's immigration laws by passing rules and regulations and actively carrying them out.
The regulations are usually more expansive, detailed versions and interpretations of Congress's laws, created by the agencies that administer those laws. They must be followed like law, but they can't be inconsistent with Congress's laws.
The federal administrative agencies that issue rules and regulations that affect immigration the most are the Department of Homeland Security (which includes U.S. Citizenship and Immigration Services or USCIS), the Department of State, the Department of Justice, the Department of Labor, and the Department of Health and Human Services.
A change in the way an agency handles a case or interprets a law or regulation can be very important. Sometimes those policies are put in writing when the agency issues a policy memorandum.
In recent years, U.S. presidents have become frustrated with Congress's inability to pass immigration legislation, so they've tried to alter enforcement of the law by issuing "executive orders." Executive orders tell the administrative agencies what to do, and they're followed like a law.
An example of a "law" that was made by executive order was the Deferred Action of Childhood Arrivals (DACA) program, which allowed certain persons who came to the U.S. as children to apply for work authorization and protection from deportation.
Executive orders are not a great way to permanently change immigration law, because the next president can issue an executive order reversing them, like President Trump did with DACA. President Trump also issued numerous executive orders concerning immigration, many of which were later halted by the courts or reversed by President Biden.
No matter how well Congress writes a law, it can't cover or anticipate every possible situation that might come up. The federal courts end up making a lot of law just by giving their opinion on what Congress meant when it passed a law.
Therefore, a single individual might be able to change U.S. immigration law just by challenging an existing law, or the government's interpretation of it, in court. The same is true with regard to rules, regulations, and policies of administrative agencies—these can be challenged by an individual and changed by the court.
One interesting feature of the U.S. legal system is that many challenges to immigration laws, rules, regulations, and policies are decided by the administrative agencies themselves, rather than by judges who are part of the judicial branch of government. Congress has said that in certain instances, there is no way to appeal the final decision of an administrative agency. In other instances, judges are bound to follow the agency's interpretation of the law even if they disagree. All of this restricts a federal judge's ability to change the law in certain areas.
State and local laws affecting immigration usually are struck down by the courts, because they can't interfere in an area like immigration that is governed by the federal government.
For example, when the state of Arizona enacted a law that made it a crime for "an unauthorized alien to knowingly apply for work, solicit work in a public place or perform work as an employee or independent contractor" in Arizona, the U.S. Supreme Court said Arizona couldn't do that because federal law already covered unauthorized employment of noncitizens.
Likewise, when a city in Pennsylvania passed an ordinance prohibiting employment of unauthorized noncitizens and precluding them from renting housing within the city, a federal appeals court struck down the ordinance because it interfered with federal immigration law.
Some state laws touching on immigration may be okay. The U.S. Supreme Court let stand an Arizona law that requires state officers to make a "reasonable attempt ... to determine the immigration status" of any person they stop, detain, or arrest on some other legitimate basis if "reasonable suspicion exists that the person is an alien and is unlawfully present in the United States." The law also provides that anyone arrested "shall have the person's immigration status determined" before being released. The Court noted that federal law doesn't prevent state police from communicating with federal immigration authorities in these situations, so unless the officers start abusing their authority by prolonging detention to check immigration status, the law can continue.