As a mere TSA screener I am not privy to much of the extra-checkpoint security. But from what I have picked up from watching and asking questions here it is.

The security starts before you enter the airport. You will never be aware of it (I wasn't even for the first couple years working at the airport).
There are lots of MWAA plainclothes police inside and outside the piers. Inside any of the piers, at any time, you will have lots of armed federal agents (FAM's, FBI, DEA, US Marshals, SS, ICE, U.S. Postal Inspectors, DSS, et al.), as well as other armed LEO's on official business. And as critic Joe Trento correctly points out, the TSA is top heavy with former secret service. This is triply true at DCA, and so you have former retired Secret Service agents (including Presidential detail) now working as TSA managers (from the DFSD on down) watching over the pier checkpoints (the guys in suits you'll see walking around the checkpoint). In addition to this, there is the extensive use of cameras that are controlled (direction & zoom) from several sites, including a remote command center.

Lastly, I would not dismiss the TSA BDO's or SPOT officers who are profiling out front of the checkpoint. One of the largest employers of military veterans is the TSA and some of the profilers at DCA worked security operations in Afghanistan and Iraq in the U.S. Army or Air Force.