Governors could be opening their states to costly court challenges as they try to block Syrian refugees from settling in the United States after the Islamic State claimed responsibility for terrorist attacks in Paris.

About two dozen governors  — mostly Republicans — moved swiftly Monday to say they would buck the Obama administration and prohibit Syrian refugees from settling in their states. That backlash was met with fierce resistance from groups such as the American Civil Liberties Union and legal scholars.

“Once a person has been granted refugee status and admitted into the United States as a refugee, a state could not ban refugees from living there without a major constitutional problem,” said César Cuauhtémoc García Hernández, a visiting assistant law professor at the University of Denver.  “That litigation would obviously take time and funding, though.”

Millions of refugees have fled Syria to nearby Middle Eastern countries and Europe amid a long-running civil war that has left the country and its economy in shambles. Authorities in France say a Syrian passport was found on a suicide bomber who targeted the national stadium outside of Paris, prompting fears that a terrorist, posing as a refugee, could wreak havoc inside U.S. borders. 

In a post on Twitter, the ACLU said it “violates the Constitution for a governor to bar an entire group of refugees from coming into their states because of their nationality.” 

Naomi Steinberg, director of Refugee Counsel USA, agreed that states do not have the authority to decide if Syrian refugees will go there. “You cannot pick out and say we are going to settle refugees from countries x-y-z and not from other countries,” Steinberg said.

But that isn’t stopping more than a dozen U.S. governors — at least for now.

The governors of Alabama, Arizona, Arkansas, Georgia, Florida, Illinois, Indiana, Kansas, Louisiana, Michigan, Mississippi, Massachusetts, North Carolina, Ohio, Texas and Wisconsin were among those who pledged to bar Syrian refugees from relocating to their states — or asked the Obama administration to pause the program. The list of states is expected to grow. 

New Hampshire Gov. Maggie Hassan, who is now running for Senate, was the first Democratic governor to support a halt on Syrian refugees pending a security review.

“Given the tragic attacks in Paris and the threats we have already seen, Texas cannot participate in any program that will result in Syrian refugees — any one of whom could be connected to terrorism — being resettled in Texas,” said Texas Gov. Greg Abbott, a Republican, in a letter to Obama. “Effective today, I am directing the Texas Health & Human Services Commission’s Refugee Resettlement Program to not participate in the resettlement of any Syrian refugees in the State of Texas.”

The outcry from governors began this weekend with Republicans Robert Bentley of Alabama and Rick Snyder of Michigan saying they would turn Syrians away after the world was horrified by the coordinated attacks on Nov. 13 in Paris that left 129 people dead.

There are currently 31 GOP governors, 18 Democrats and one independent.

“We must take immediate action to ensure terrorists do not enter the nation or our state under the guise of refugee resettlement,” said Kansas Gov. Sam Brownback, a Republican. 

The cascade of governors working to block Syrian refugees underscores the pressure elected officials feel from their constituents.

“Once you have a great deal of governors opting out of participating, the pressure continues to build on the remaining governors,” said Rory Cooper, a former aide to then-House Majority Leader Eric Cantor, R-Va.

Obama said Monday refugees — including those from Syria, where the group known as ISIS or ISIL has a stronghold — are subject to rigorous screenings and security checks.

“We also have to remember that many of these refugees are the victims of terrorism themselves,” Obama said during a news conference in Turkey, where he is attending the G20 summit. “That’s what they’re fleeing. Slamming the door in their faces would be a betrayal of our values.”

States that try to divert federal refugee resettlement funding away from Syrian refugees could also face legal troubles, according to Greg Chen, director of advocacy for the American Immigration Lawyers Association. 

The Department of Health and Human Services’ Administration for Children and Families provides funding for state refugee resettlement programs, which is then passed down to nonprofit groups that help with placing refugees.

Chen said states that try to restrict funding may be violating federal contracts and existing arrangements with nonprofits. 

“If the state tries to say ‘we’re not going to give this money to Syrian refugees,’ it’s very possible they’re [also] in violation of anti-discrimination laws,” Chen said.  

Still, states with this policy could foment a sense of ill will toward migrants and cause concrete concerns. 

“Even the private resettlement organizations would likely feel pressured to avoid resettling refugees in places they were not wanted, especially because they rely heavily on cities providing a warm welcome,” García Hernández said.