Democrats are speaking out about President Donald Trump's threat to bus illegal immigrants to sanctuary cities, and many are none too pleased. Still, some leaders insist they'll welcome anyone with open arms.

What are the details?

Last week, the president announced on Twitter, "Due to the fact that Democrats are unwilling to change our very dangerous immigration laws, we are indeed, as reported, giving strong consideration to placing illegal immigrants in sanctuary cities only. The radical left always seems to have an open borders, open arms policy — so this should make them very happy!"

Yet several Democrats are not, in fact, happy. Particularly in California — home to many longtime "sanctuary cities" — which has statewide laws in place to protect illegal immigrants from deportation.

Oakland Mayor Libby Schaaf (D) called the Trump administration's policy idea an "outrageous abuse of power" and characterized it as un-American. The president called Schaaf out on Twitter, saying it was "so interesting to see the Mayor of Oakland and other sanctuary cities do not want our currently 'detained immigrants.'" She responded by insisting her city "welcomes all, no matter where you came from or how you got here."

California Gov. Gavin Newsom (D) also pushed back against the Trump threat. According to the Los Angeles Times, Newsom "insisted the federal government is already 'dumping people' in California by processing migrants legally seeking asylum at the border and releasing them into California before they have time to connect with their sponsors in the U.S."

House Speaker Nancy Pelosi's (D) office also weighed in, with spokeswoman Ashley Etienne saying in a statement, "The extent of this Administration's cynicism and cruelty cannot be overstated. Using human beings — including little children — as pawns in their warped game to perpetuate fear and demonize immigrants is despicable, and in some cases, criminal," USA Today reported.

Democratic presidential candidate Sen. Cory Booker (N.J.) accused Trump of "trying to pit Americans against each other and make us less safe," with the proposal.

House Democrats are now formally investigating the administration's plan, with the chairmen of the Judiciary, Oversight, and Homeland Security Committees requesting pertinent documents and communications from the White House and Department of Homeland Security, ABC News reported.

House Judiciary Chairman Jerry Nadler (D-N.Y.) told CNN Sunday, "The president has no right to spend money appropriated by Congress for other purposes to ship immigrants all over the country."

New York City Mayor Bill de Blasio (D) vowed to sue President Trump over the threat to ship illegal immigrants to his city, calling the administration's plan itself, "just plain illegal."

Anything else?

In spite of the pushback, some sanctuary city leaders have appeared to call the president's bluff, insisting they're unfazed by the prospect of his proposal. Seattle Mayor Jenny Durkan (D) responded to the plan on "Hardball with Chris Matthews" Monday, saying, "It's not a threat to us," and explained that her city and others across the country are "ready to stand up" and take folks in.

Durkan explained, "Just over this weekend, I've received hundreds of emails from people in our area offering places of refuge and saying 'I'll take some in my own home; I'll take some in our church. I think people will step up."

When pressed by Matthews as to whether her city could take in 50,000 of the approximately 1 million people seeking asylum in the U.S., Durkan responded, "I don't think we'd take 50,000 in one go, but we've taken thousands of people in Seattle."