That earned him a reprimand from Democratic Sen. Mark Warner of Virginia, who tweeted: "Federal employees don't go to work wearing red or blue jerseys. They're public servants." Roughly federal 420,000 workers were deemed essential and are working unpaid, unable to take any sick days or vacation.

An additional 380,000 are staying home without pay. While furloughed federal workers have been given back pay in previous shutdowns, it's not guaranteed. The Senate passed a bill last week to make sure workers will be paid. The House will probably follow suit.

The longer the shutdown lasts, the more government activities will grind to a halt. It's already caused a lapse in money for nine of 15 Cabinet-level departments and dozens of agencies, including the departments of Homeland Security, Transportation, Interior, Agriculture, State and Justice.

Many national parks have closed while some have limited facilities. The National Flood Insurance Program announced it will no longer renew or issue policies during the shutdown.

"I think it's obvious that until the president decides he can sign something — or something is presented to him — that we are where we are," said Sen. Pat Roberts, R-Kan., who opened the Senate for the minutes-long session. "We just have to get through this."

House Democrats tried Thursday to offer a measure to re-open government, but they were blocked from action by Republicans, who still have majority control of the chamber until Democrats take over Jan 3.

"Unfortunately, 800,000 federal workers are in a panic because they don't know whether they'll get paid," said Rep. Jim McGovern, D-Mass., who tried to offer the bill.