Even when appropriations run out, employees whose activities “directly relate to preserving the safety of human life or the protection of property” must keep working. The nature of DHS’s work—under its vast umbrella are agencies like the Secret Service, the Coast Guard, and Customs and Border Protection—means that most DHS employees are in that category. Here’s a selection of what DHS does in a typical day, per the department’s website:

Screen approximately 2 million passengers and their checked baggage before they board commercial aircraft … Train 3,400 federal officers and agents from 89 different federal agencies ... Inspect more than 47,000 truck, rail and sea containers … Seize 11,435 pounds of narcotics at or near ports of entry nationwide … Monitor 1,428 radiation portal monitors to scan 100 percent of all containerized cargo entering from Canada and Mexico … Make an average 728 administrative arrests and 638 criminal alien arrests

87 percent of the approximately 230,000 DHS employees would keep working in the event of a shutdown. They just wouldn’t be paid—unless they’re part of the tiny group whose salaries come from sources other than the standard appropriations bill. All employees would almost certainly receive back pay, as they did after the 2013 shutdown, but no one would see a check until funding is restored. For a group that’s reported having the lowest morale of any large government agency, that could be a problem. (Evidently TSA agents don’t like airport lines either.) Nor would those working men and women have the support of more than 30,000 furloughed “non-essential” colleagues, barring an emergency recall for a specific project. Homeland Security secretary Jeh Johnson has said his administrative staff at headquarters would be “dialed back to a skeleton.”

The shutdown would touch a host of programs: Law enforcement civil-rights training would cease; FEMA flood-risk mapping and DHS’s chemical site regulatory program would stop. There would be disruptions to procurement and hiring processes, like finding the new Secret Service agents needed for the coming presidential election cycle. Civil-liberties complaint lines and investigations would be curtailed. Anything funded by a DHS “non-disaster” grant would stop too, from surveillance cameras in New York to K-9 units in Massachusetts to firefighters’ oxygen tanks in Denver.

In the context of a showdown over immigration, it’s also interesting what wouldn’t be shuttered. The House Appropriations Committee admits that the Citizen and Immigration Services agency, which processes applications from immigrants seeking protection under Obama’s program, is “entirely self-funded” through application fees. That’s why the GOP has attached language to past DHS funding bills explicitly stopping the deportation deferrals, and why Democrats have prevented the passage of such provisions. (FEMA disaster relief operations and national flood insurance are also funded through fee revenues and other sources.)