It comes amid reports that the units have been told to prepare for the potential use of improvised explosive devices (IEDs) by people plotting to attack the Capitol in the days around the inauguration.
Politico cited two guardsman briefed this week, who were told that IEDs planted last week at the Republican National Committee and the Democratic National Committee headquarters were not thought to be an isolated incident.
Reporters arriving to cover the impeachment hearings in the House of Representatives found hundreds of troops bedded down in the visitor centre, akin to an “armed garrison” as one described it.
“Many are cuddling their firearms, fatigues over their heads to block light, and riot gear in neat piles,” Newsy reporter Nathaniel Reed tweeted, noting that streets around the complex are largely blocked.
The Capitol and all surrounding buildings are completely sealed off with anyone requiring access to the site needing to show staff ID to everyone they encounter as they pass through checkpoints and the newly installed 10 foot fencing.
In the visitor centre, a large statue depicting Freedom looks down on dozens of troops napping on the huge expanse of floor below.
Outside, racks and racks of rifles and riot shields stand ready for the protection of Congress during the impeachment process.
“The US Capitol is now a military base. Massive show of force outside. National Guard is armed with rifles and spread just few feet apart along perimeter, as House prepares to impeach the president,” tweeted Bloomberg’s Eric Wasson.
Some troops took the opportunity to explore their new barracks, with one group of Black troops seen having their photo taken with the statue of Rosa Parks, while another group was given a tour by Representative Brian Mast, a veteran.
Inside, Lawmakers must now go through metal detectors to get onto the house floor — a measure protested by a number of Republican lawmakers.
Law enforcement and the military are responding to a number of threats to the US government following the assault on the Capitol by pro-Trump rioters a week ago.
Five people died in the violence including a Capitol Police officer. The article of impeachment being voted on by Congress is that the president incited the insurrection.
The FBI and Department of Justice are so far pursuing more than 170 cases against those that stormed the building — a figure that is likely to climb into the hundreds.
Charges of conspiracy and sedition are expected against some of the rioters, which could mean up to 20 years in prison.