Unless Congress extends its funding by Friday, the Department of Homeland Security will shut down. Driving the crisis are President Obama’s 2012 and 2014 executive orders that protected millions of undocumented immigrants from deportation. Republicans, who oppose the orders, set up this month’s showdown by supporting a bill in December that only funded DHS—the agency which will carry out those executive orders—for a few months. The Republican-dominated House then passed a DHS funding bill halting the immigration measures in January, but Democrats have filibustered four times this month to prevent its passage in the Senate. The result is another congressional standoff, this one complicated by a federal judge’s ruling against the order in Texas and the possibility of legislators punting with a short-term bill.

But what does DHS actually do? And what happens if the agency isn’t funded?

The agency whose mission is “to ensure a homeland that is safe, secure, and resilient against terrorism and other hazards” hasn’t run out of funds since … 2013, when the entire federal government crashed to a halt in a dispute over Obamacare. On the bright side, there’s now a lot of data on the effects of a shutdown—and at the DHS, less shuts down than one might think.  

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.)

In fact, the only immigration-related program that would actually shut down is E-Verify, the electronic system that checks the work eligibility of new employees. Just 500,000 of roughly 6 million private employers use the service, but because of its potential, expanding it was a key feature of the comprehensive immigration reform bill that passed the Senate in 2013. Ironically, a confrontation that started because Republicans opposed new protections for illegal immigrants may make it easier for illegal immigrants to find work.

Politicians have debated the significance of it all. As Friday’s deadline approaches, Senator Lindsay Graham, Republican of South Carolina, has worried that a shutdown would “add gasoline to the fire” of the terrorist threat, while GOP Representative Thomas Massie of Kentucky correctly notes that “a lot of things are supposedly funded anyway.” Other Republicans are simply weighing whether confronting Obama over executive action is worth the possible damage to the party’s reputation. Secretary Johnson is, of course, unlikely to undersell the importance of his own department, but it’s hard to disagree with his contention that Congress can’t shut down the Department without substantively damaging national security. Though most DHS employees would keep working, they wouldn’t have the full range of tools and support they need to do their job.

Washington should have a fair idea of what will happen in a shutdown. It just doesn’t yet know if a shutdown will happen.