President-elect Donald Trump is vowing to block federal funding to Sanctuary cities, but the incoming commander-in-chief may find it tougher to take on another powerful protecter of illegal immigrants -- sanctuary churches.

Hundreds of houses of worship around the nation have pledged to provide safe harbor to illegal immigrants facing deportation, as have cities such as Chicago, Los Angeles, Philadelphia and New York City. While Trump may relish a policy and funding fight with the leaders of Democratic strongholds, imposing his administration’s will on churches could be another matter.

“There is a long tradition in American law enforcement of not breaking into churches in order to arrest someone unless the person is wanted for an act of violence,” Fox News Senior Judicial Analyst Judge Andrew Napolitano told FoxNews.com. “In order for the police or for Immigration and Customs Enforcement to enter a church against the will of a priest or minister who is running the church, they need an arrest warrant for a specific human being –short of that, they’re not going to go in there.”

In New York City alone, there are 11 congregations that offer sanctuary for illegal immigrants, and according to the Rev. Donna Schaper, senior minister of Judson Memorial Church, more could follow.

“There has been a tremendous increase in interest since the election,” Schaper told FoxNews.com. “The newly elected president is threatening to deport many of them and they want to be safe—churches have a moral mandate to help people in a way that is different than cities in general.”

Schaper is a founder of New Sanctuary Movement New York, an interfaith network of congregations that help to provide spiritual, financial, emotional and legal support to illegal immigrants. In some cases, those under the church’s protection move into the building and live there indefinitely.

Earlier this month, Javier Flores, a Mexican and father of three who has been deported multiple times, appeared at a news conference with clergy from Philadelphia’s Arch Street United Methodist Church, where he has been offered sanctuary.

“Today and every day, if Javier and his family choose to stay with us, they will have a home with us,” the Rev. Robin Hynicka told Religion News Service.

Since Trump’s Nov. 8 triumph over Hillary Clinton, the New Sanctuary Movement Philadelphia has been inundated with congregations clamoring to sign up, said Executive Director Peter Pedemonti.

“In Philadelphia, there is a continued wave of support for faith communities to stand up and say that deporting people and separating families is a violation of our faith values,” Pedemonti told FoxNews.com. “These policies call us to feed on the worst of humanities –our fears and divisions, and our role, as a faith community is to bring us to our best selves.”

Some advocates for illegal immigrants say it was President Obama’s aggressive deportation policies that first boosted the sanctuary church movement.

“We have already seen the deportation machine at work with over 2.5 million people deported under the Obama Administration,” Church World Service National Grassroots Coordinator the Rev. Noel Andersen said. “The keys to that machine are now being handed over to an Administration that has shown in their campaign rhetoric vitriolic language, and I think congregations across the board have been concerned about this.”

Trump has been resolute about his plans to build a wall on the Mexican border, but has sent somewhat mixed signals when it comes to deporting illegal immigrants. At times during his campaign, he said all illegal immigrants must go, but more recently, he has said the focus will be on those who have been arrested for alleged crimes.

If churches choose to offer safe harbor, federal agents will in most case avoid arresting, searching or interviewing people inside them. The tradition Napolitano alluded to is official police of ICE, which treats churches as “sensitive locations,” said spokeswoman Jennifer Elzea.

“DHS is committed to ensuring that people seeking to participate in activities or utilize services provided at sensitive locations are free to do so without fear or hesitation,” Elzea said.

There are limits to the protection a church can provide, however. Napolitano said illegal immigrants not accused of any subsequent crime cannot be pursued into churches. But no church can shelter a dangerous criminal.

“If the person is a fugitive from justice or has escaped from jail or is hiding from a search warrant, then yes – the church is breaking the law by shielding him,” Napolitano said. “But if he is not a fugitive from justice, and just living here illegally, it is not a crime for a church to give him food, shelter, or clothing.”

One member of Trump’s Evangelical Advisory Board told FoxNews.com Trump has no intention of carrying out mass deportations.

“The Trump Administration wants to fix the immigration system,” said Tony Suarez, executive vice president of the National Hispanic Christian Leadership Conference. “They want to deport terrorists and criminals –I don’t know of anyone who can argue with that.”