A shutdown of the Department of Homeland Security would cause "terrible disruption" to average Americans at airports, customs offices and border crossings, Secretary Jeh Johnson said Tuesday.

Johnson spoke hours after Senate Democrats blocked a bill that would have funded the department but blocked implementation of President Obama's plan on immigration reform. Republicans, upset with the the president over an executive action that would let as many as 5 million undocumented immigrants stay in the country, have only funded the department through Feb. 27.

"A shutdown of the Department of Homeland Security in these times is frankly too bitter to contemplate, but we have to contemplate it," Johnson said. Beginning March 1, essential personnel in the department would still have to show up for work, but they wouldn't receive a paycheck. "It is horribly unfair to ask people in the critical role of Homeland Security to come to work and not get paid because Congress can’t fund the department," Johnson said.

Johnson said that he's still optimistic that Congress will reach a deal on Homeland Security before the money runs out. He then listed the areas that would have to be curtailed if his department's funding stopped. "The department interfaces with the American public more than any other department through the Transportation Security Administration, through customs, through immigration, through our ports," he said. "So to even contemplate a shutdown means contemplating cutting back on things that are vital to Homeland Security and causing terrible disruption with the American public right now."

What does immigration have to do with funding for the Department of Homeland Security? According to some Republicans, a whole lot. Here’s a breakdown of what’s happening in the fight to keep the DHS budget running. (Pamela Kirkland/The Washington Post)