SAN FRANCISCO One of President Donald Trump s first major executive actions on immigration policy is facing massive political blowback and will almost certainly crash and burn under the Constitution once courts begin to scrutinize the fine print.

During a visit to the Department of Homeland Security on Wednesday, Trump signed an executive order aimed at strong-arming so-called sanctuary cities into cooperating fully with his efforts to ramp up deportations. Threatening loss of federal funding and using shaming tactics for localities that refuse to comply, the order is styled as a call to obey existing immigration laws even though immigration experts and civil liberties groups are doubtful Trump even has the constitutional authority to enforce it.

Independent of the ultimate legality of the executive order, politicians from those sanctuary cities say they arent budging, and legal advocacy groups are gearing up for the coming legal fight.

The president is in for one hell of a fight, California state Sen. Scott Weiner (D), who represents San Francisco, said in a statement.

Boston Mayor Marty Walsh (D) said his city will not retreat one inch from its policy against holding undocumented immigrants it otherwise would not hold based on requests from Immigration and Customs Enforcement, and Seattle Mayor Ed Murray said his city will not be intimidated by the authoritarian message coming from this administration. San Francisco Mayor Ed Lee (D) said nothing has changed in his city, noting the lack of specifics in Trumps order.

We are going to fight this, and cities and states around the country are going to fight this, New York Mayor Bill de Blasio (D) said at a press conference Wednesday.

New York Attorney General Eric Schneiderman (D) already began hinting at a legal challenge, releasing a statement that Trump lacks the constitutional authority for his executive order and that he will do everything in [his] power to push back if the president does not rescind it.

Washington state Attorney General Bob Ferguson (D) also warned of potential legal challenges to come, saying in a statement that the order raises significant legal issues that my office will be investigating closely to protect the constitutional and human rights of the people of our state.

Theres no exact definition of sanctuary city. Places like San Francisco and New York use the term broadly to refer to their immigrant-friendly policies, but more generally the term is applied to cities and counties that do not reflexively honor all of Immigration and Customs Enforcements requests for cooperation. Many of these localities do work with ICE to detain and hand over immigrants suspected or convicted of serious crimes, but they often release low-priority immigrants requested by ICE if they have no other reason to hold them.

The reason that many local law enforcement officers dont honor detainers is because courts have said that they violate the Constitution, and if they violate the Constitution, the localities are on the hook financially, said Cesar Cuauhtemoc Garcia Hernandez, a law professor at the University of Denver who teaches on the intersection of criminal law and immigration.

Just on Tuesday, a federal court in Rhode Island joined several others that have ruled in recent years that certain ICE detainers can violate peoples constitutional rights even those of U.S. citizens.

But Trumps executive order seems to overlook this legal reality, and instead frames sanctuary cities with the alarmist rhetoric he used on the campaign trail.