There are now 340 sanctuary cities — and the list is growing. All of them choose to ignore federal immigration law by refusing to report detained undocumented immigrants to federal authorities under most circumstances.

Partly as a result, deportations of those who entered the U.S. illegally are at a 10-year low — even according to the Obama administration’s new rigged redefinition of deportation as also occasionally preventing illegal entry at the border.

Some of the 1,000 undocumented immigrants who go unreported to federal authorities each month and are thereby shielded by sanctuary cities from deportation have been accused of violent crimes. According to a new report by the Center for Immigration Studies, more than 2,000 of the immigrants released have used their freedom to commit crimes.

Last year, San Francisco alone released from its custody 252 undocumented immigrants whom federal authorities had asked the city to hold, according to the report. Most notoriously, the city protected Juan Francisco Lopez-Sanchez — five times previously deported, seven times previously convicted of felonies — who once free allegedly murdered 32-year old Kathryn Steinle in front of witnesses.

Steinle’s tragic fate is not unique. And the Obama administration’s record on illegal immigration is little better than that of sanctuary cities. The Department of Homeland Security has told Congress that from 2010 to 2014, U.S. Immigration and Customs Enforcement released 121 undocumented immigrants who had committed crimes and were later charged with homicide.

In 2013 alone, ICE released more than 36,000 undocumented immigrants with criminal convictions. One thousand of them were charged with committing subsequent crimes, according to the Center for Immigration Studies report.

Why have these sanctuary cities and an agency overseen by the Obama administration chosen to disregard federal law and risk the safety of the public?

First, a Republican-majority Congress is unlikely to repeal present immigration statutes. That means an amnesty agenda must be carried out in defiance of the law by city authorities sympathetic to illegal immigration, with a wink and a nod from the Obama administration.

Second, the Obama administration presumably envisions minorities voting in bloc fashion and hopes the Democratic Party will be so rewarded at election time.

Third, cities are generally more liberal than the country at large. There are no political downsides for high-ranking city officials who choose to disregard law, but lots of advantages in appeasing liberal constituencies.

In the 19th century, the Supreme Court issued a number of rulings prohibiting particular states from ignoring federal laws — from unpopular tax policies to the establishment of Native American reservations.

The most prominent nullificationist was Sen. John C. Calhoun, a South Carolina states-rights advocate and the spiritual godfather of sanctuary cities. Calhoun declared, for example, that federal tariffs should not apply to his state.

Apparently, sanctuary cities do not understand the illiberal pedigree of federal nullification, which was at the heart of the Confederate secessionist movement of 1861. In the 1960s, segregationists declared that Supreme Court decisions and integration laws did not apply to their states. In some states, local law enforcement refused to cooperate with federal authorities to integrate schools.

What would San Franciscans do if conservative counties and towns followed their lead? Perhaps a rural Wyoming sheriff can now look the other way when he spots a cattleman shooting a federally protected grizzly bear or predatory timber wolf — or at least shield the cattleman from federal officials. Should public schools in Provo, Utah, start the day with school-wide prayers?

The mayor and sheriff of sanctuary-city San Francisco are kindred spirits with Kentucky county clerks who want to opt out of licensing gay marriages. Following the lead of elected Rowan County clerk Kim Davis, other Kentucky clerks have vowed that they will not issue gay marriage licenses, all too happy to nullify a Supreme Court decision.

Sanctuary cities remind us that the obstacle to supposed comprehensive immigration reform is not opposition from intolerant conservative bogeymen.

Many Americans support a pathway to legal residence for undocumented immigrants currently living in the United States — if the border is first closed to further illegal immigration, if legal immigration is made ethnically blind and predicated on merit such as education and skills, if undocumented immigrants pay a fine and meet residency requirements, if applicants for legal residence are neither on public assistance nor have committed crimes, and if those with criminal records and without work records are sent back to their countries of origin.

In contrast, sanctuary cities refuse even to inform federal authorities about the undocumented immigrants with felony convictions who are residing in their jails.

So where do we go from here?

If immigration law were nullified, almost anyone could enter the United States. Perhaps undocumented immigrants from Asia would soon outnumber those from Mexico and Central America. And if cities can declare supposedly conservative federal immigration law invalid, then some states might do the same, deeming lots of federal statutes too liberal.

I thought the Civil War ended these dangerous ideas for good. Apparently not.

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