Oakland's liberal mayor attacks Trump over immigration policies. Trump swings back with defiant challenge.

President Donald Trump and Oakland Mayor Libby Schaaf (D) traded barbs on social media Saturday over the president's controversial immigration policies.

What's the background?

Last week, the Washington Post revealed top White House aides once considered releasing detained migrants in "sanctuary cities," which are almost exclusively Democratic-led progressive cities. Trump later confirmed he is considering enacting the policy.

In fact, White House deputy press secretary Hogan Gidley told Fox News' Jeanine Pirro Saturday that the administration is actively working with Homeland Security and Immigration and Customs Enforcement officials to begin sending illegal immigrants to sanctuary cities.

"They have said they wanted all of these illegal aliens into their communities," Gidley said. "We're working with DHS, we're working with ICE, to try and make sure that happens because after all, it's what they want. They should not say 'This is retribution politically,' they should say, 'This is an olive branch.'"

What happened between Trump and Schaaf?

After the Washington Post revealed the proposed policy, Schaaf, in an interview with KTVU-TV, called the policy idea an "outrageous abuse of power," characterized it as un-American, and said the Trump administration essentially proposed using human beings as "instruments of political payback."

In a separate interview with CNN, Schaaf similarly denounced the proposed policy, but added that she is "proud to be the mayor of a sanctuary city."

Trump hit back at Schaaf on Saturday, questioning why non-sanctuary cities should be forced to carry the burden of caring for illegal immigrants when sanctuary cities already welcome them.

Schaaf swung back, accusing the president of "fanning hate and division."

About 30 minutes later, Trump responded by declaring the legal right to release illegal immigrants in sanctuary cities while demanding, perhaps tongue-in-cheek, that California, a liberal safe haven, accept them.

Schaaf did not respond to Trump's last tweet.