Following up on repeated campaign promises, the Trump administration has started pursuing the mass deportation of the estimated 11 million undocumented immigrants present in the United States. Some upcoming proposed measures include workplace raids, military involvement, and the construction of federal “staging” camps for detainees awaiting deportation.

To lay the groundwork for mass deportation, the administration has already issued a series of executive orders to declare a national emergency at the border and to utilize the military for border enforcement against migrants entering the country, including those seeking asylum at designated ports of entry. Skipping the traditional GOP dogma of fiscal conservatism, Trump has said there will be no budgetary limits on the scope of his mass deportation efforts.

Not content to only target undocumented immigrants, the administration has also suggested it will strip legal protections from large numbers of people who are living legally in the country with temporary protected status. Rather than take heat for separating families this time, border czar nominee Tom Homan said they plan to remove U.S.-born citizen children alongside their undocumented parents. To facilitate removing those children, they’ve issued an executive order to eliminate the Fourteenth Amendment’s birthright citizenship provision, which confers citizenship automatically to every person born inside the country. That executive order, which purports to withhold citizenship from U.S.-born children of immigrants unless at least one parent is a permanent resident, is already being challenged in court by 22 state attorneys general.

Stopping this mass deportation regime is unlikely at the federal level, given that Trump and his GOP loyalists will control the House, the Senate, and the White House for at least the next two years. In fact, the new Republican-controlled Congress—with the help of some Democrats—is already at work trying to expand the scope of deportable offenses and conditions for mandatory detention. Therefore, any governmental roadblocks will have to come from the states.

States can’t bring Trump’s mass deportation agenda to a grinding halt; under the Constitution, federal authority over immigration law is exclusive, since allowing states to set 50 different standards would wreak havoc on foreign policy. However, states can make mass deportation much more difficult. By refusing to share resources with the federal government, denying the use of their National Guard for enforcement, suing to prevent military policing in their jurisdiction, bolstering state law protections against unreasonable searches and seizures, and providing key resources to U.S.-born citizens, state officials can prevent—or at least delay—some of the worst excesses on deck.

The main thing state governments can do to frustrate mass deportation is to simply refuse to assist federal authorities. Under current Supreme Court precedent, states don’t have to allow their resources to be “commandeered” for federal purposes. This rule was established in the context of conservative states opposing federal gun background check legislation and preventing their law enforcement from running those checks, leaving enforcement to federal authorities. In the first Trump administration, a number of “sanctuary” jurisdictions relied on this anti-commandeering principle to deny their assistance. The administration’s response was to withhold DOJ grants from those states, leading to a spate of federal litigation on whether those funds could be withheld. But before the Court could hear those cases, the Biden administration took power and ordered the grants reinstated, rendering the question moot.

In the immigration context, states can use the anti-commandeering doctrine in several ways. First, they could simply not share information with federal authorities about undocumented immigrants. This would include immigrants who apply for identification cards, participate in state assistance programs, or attend public schools. They can also withhold information about contacts with suspected undocumented people, including witnesses to crimes and even those arrested for crimes.

States can also refuse to honor immigration detainer requests, where Immigration and Customs Enforcement asks a state to hold a suspected undocumented person in custody beyond their term of incarceration. These requests arise when an immigrant in state custody is set to be released—either at the end of their sentence or, more commonly, after they have posted bail following an arrest. But these detainer requests are just an ask by ICE; they are not legally binding, and state and local law enforcement agencies can decline them.

Finally, states can deny the use of their detention facilities for federal immigration purposes. During the first Trump term, the administration often leased space in local jails and prisons for immigration detention; in response, a number of states passed laws to refuse the use of their facilities for this purpose. The Biden administration turned to private prison facilities for immigration detention—as of 2023, approximately 90 percent of ICE detainees were held in private facilities—but Trump’s agenda will result in increased demand for detention space beyond what even the private prison industry can produce on relatively short notice. States who do not share Trump’s desire to deport vulnerable residents should reaffirm their commitment to withholding their state-run detention resources from ICE.

States can also refuse to let their national guards be used for civilian policing activities, since national guards are under the exclusive command of state governors. Federal law authorizes the federal government to “federalize” national guard troops in some circumstances—most famously in the case of the Little Rock Nine, when President Dwight Eisenhower mobilized the Arkansas National Guard to prevent violence against Black students desegregating public schools in the state capital. Trump has already ordered the use of National Guard for border enforcement, and he may try to extend federalization to domestic operations, too. Until he does so, however, governors can withhold the units under their command.

The administration has also floated the idea of using national guards in Republican-controlled states to mobilize against neighboring states, a notion that should trouble anyone who remembers the Civil War. If the administration seeks to do so, Democratic-controlled states would seek injunctions in federal court under the Posse Comitatus Act. That Act, which dates to Reconstruction, forbids the military, whose troops are not trained in law enforcement, from conducting domestic policing.

To be effective, of course, this would require the cooperation of federal judges in a branch of government already dominated by Trump appointees. But it is the strongest tool available to state governments, since the concept of armed soldiers going through civilian populations demanding papers is historically not a sign of robust civil liberties.

A core principle of federalism is that states can interpret their constitutions to offer stronger protections than the U.S. Constitution, including protections against unreasonable searches and seizures. For example, Pennsylvania requires a higher threshold for reasonable suspicion than required under federal Fourth Amendment case law. By limiting the number of people identified by law enforcement, states can reduce touchpoints where immigrants can come to ICE’s attention. They can also explore policies like eliminating cash bail to keep immigrants out of detention for prolonged periods; dealing with low-level offenses through fines and ticketing that don’t require court appearances; and keeping immigrant witnesses anonymous when their testimony will not be required at trial.

Trump’s enablers in the federal legislative and judicial branches are not going to stop Trump’s plans for unprecedented mass deportation. Nor will officials in Republican-controlled states who are eager to assist with that project. The only bulwark against the deportation regime will be progressive states upholding their commitments to civil liberties. The Constitution entrusts states with many tools that can frustrate the administration’s worst excesses. They should use all of them.