On Dec. 21, President Donald Trump took the federal government hostage. Now he’s delivered his ransom note.

Give him $5.7 billion for a border wall and raise some new impediments to legal immigration, and the president says he’ll consider letting federal employees go back to working for pay.

It’s an outrageous ploy and he shouldn’t get away with it. But Sen. Susan Collins does not see it that way.

On Wednesday, the Maine Republican endorsed Trump’s proposal, calling the power play a “compromise.”

Collins and everyone else should know why you don’t negotiate with hostage takers. Trump could reopen the government this week and shut it down again in September after issuing another non-negotiable demand. The correct response from members of Congress, regardless of party, should be to reopen the government — over the president’s veto if necessary — before any horse-trading on policy begins.

Collins’ support of Trump is inconsistent with previous votes and statements. She joined a unanimous Senate on Dec. 19 that passed a spending bill without any border-wall funding. But what had looked like an agreement was scuttled by the president after he was shamed into opposition by pundits in right-wing media.

Earlier this month, Collins endorsed the approach employed by the House, which called for short-term funding for all of the government agencies except Homeland Security, leaving the parties an opportunity to negotiate on border security.

The Democrats’ proposal to pass a bill like that in the Senate, scheduled for a vote Thursday after Trump’s bill. Collins said she would also vote “yes” on that bill, but stressed that the president’s bill contained “the outlines of a compromise.”

The dictionary says a compromise is the “settlement of a dispute in which each side makes concessions.” That’s not Trump’s bill.

It was reportedly cooked up in the White House by the president, vice president and the president’s son-in-law, Jared Kushner, without any give-and-take with Democrats. The administration’s “concession” is a protection from deportation for the Dreamers, young adults who grew up in America without immigration papers because they came to the country illegally as children. But unlike Collins’ bipartisan 2018 immigration compromise, which Trump first agreed to and then blew up, it’s only a temporary reprieve.

And in addition to his wall funding, Trump’s bill would to make it harder for people who are running for their lives to seek asylum in the United States, ending policies that were established in response to the slaughter of civilians in World War II.

To entice Democrats to vote for it, they’ve thrown in nearly $13 billion worth of relief for communities struck by natural disaster, the way a car dealer might dangle free leather seats as a way to close a deal. But that doesn’t turn this extortion attempt into a compromise.

Legitimate disaster relief should be funded. Law-abiding Dreamers should have a path to citizenship. And the border wall, the asylum rules and all the other aspects of an immigration system badly in need of reform should be the subject of a fair legislative process, which probably would require some concessions from both sides if anything is ever going to be accomplished.

But that process hasn’t started yet, and before it does, Congress should fund the government, pay the federal employees and keep the country’s promises to the people who depend on their services, whether they are senior citizens on food stamps or business travelers on commercial airliners.

It would be a grave mistake to reward this president for inflicting pain on innocent Americans and then using that pain as leverage to accomplish political ends.

Collins should vote “no” on Trump’s border wall bill, and then “yes” on a bill to reopen the government.