With top GOP presidential candidates Donald Trump, Ted Cruz and Marco Rubio all calling for an end to so-called “sanctuary cities,” the future of safe havens for undocumented immigrants has become a core policy issue in the upcoming election.

Sanctuary cities follow certain procedures in order to shelter undocumented immigrants from immigration officers and deportation. In general, these municipalities do not permit police or municipal employees to ask about someone's immigration status.

According to the Center for Immigration Studies, there are over 300 cities, counties and states currently considered sanctuary cities.

Random Death Fires up the Right

Awareness of sanctuary cities hit the nation on July 1, 2015, when a 32-year-old woman named Kathryn Steinle was shot and killed in San Francisco by an undocumented immigrant named Juan Francisco Lopez-Sanchez.

Upon his arrest, it was revealed Sanchez had been deported back to Mexico on five different occasions. Sanchez admitted to repeatedly coming back to the U.S. for work, and he knew San Francisco was a sanctuary city where he would be safe from immigration.

Trump, who had just announced he was running for president, immediately seized the incident as an opportunity to say that Steinle’s murder was proof that the U.S. needed to secure its border.

"Nobody else has the guts to even talk about it. The American people deserve a wall," Trump said.

Fellow Candidates Follow Trump

GOP presidential hopefuls, from Jeb Bush to Ben Carson, all came down on sanctuary cities as part of their platforms.

Marco Rubio’s official campaign site outlines his plans to cut off federal funding for sanctuary cities and allocate those funds to local governments.

Ted Cruz spoke out against sanctuary cities while talking to Fox’s Greta Van Susteren.  In a clip provided on his official site, Cruz links the problem of immigration to the actions of liberal mayors who refuse to follow the rule of law regarding the deportation of undocumented migrants.

According to Cruz, “The Democratic Party supports illegal immigration."

Democrats on Deportation

However Democrats feel about illegal immigration, both Hilary Clinton and Bernie Sanders are both for protecting sanctuary cities.

In the aftermath of Steinle's murder, Xochitl Hinojosa, a spokeswoman for Clinton’s campaign, restated the candidate’s commitment to sanctuary cities.

"Hillary Clinton believes that sanctuary cities can help further public safety, and she has defended those policies going back years," Hinojosa said.

Sanders' immigration policy makes it clear the candidate rejects recent criticism of sanctuary cities. His campaign site indicates that, if he were elected president, he would seek to strengthen relationships between immigrant communities and local law enforcement in order to respect the constitutional rights of immigrants.

Wisconsin Cracks Down on Sanctuary Cities

As Immigration Reform reports, the Wisconsin Assembly just passed legislation that would penalize local governments for maintaining policies that prevent officers from reporting the immigration status of anyone charged with a crime.

AB 450 says that if a judge finds a city in violation, state aid could be reduced by between $500 and $5,000 for each day the sanctuary policy remains in place.

The measure will now go to the Senate.

Around 20,000 Latinos gathered at the state Capitol on Feb. 18 to protest the bill. A spokesman for the Madison police department said the crowd was peaceful and respectful.

Zeus Rodriguez, a protester from the group Hispanics for Leadership, said the bill was motivated by the drastic stance against migrants on the right.

"This is the type of unnecessary red meat bill to satisfy the Trumpkins," said Rodriguez.