A320
Flight Crew Air Transport Pilot (ATP)
Flight Crew Last 90 Days 180; Flight Crew Total 16500; Flight Crew Type 3600
Conflict NMAC
Flight Crew Took Evasive Action
Procedure; Airspace Structure
On the Mount Vernon Visual on 11 mile final and level at 2500 ft; Potomac advised us of pop-up helicopter traffic 500 feet below our altitude.  Due to the potential conflict Potomac cancelled our approach clearance and said to maintain 2500 feet. We had no traffic displayed on the TCAS at that time.  A few seconds later at about 10 mile final the traffic displayed in red and we received a TCAS resolution advisory (RA) to climb. We climbed and told ATC that we were climbing due to the RA.  The 'climb' warning stopped after about 300 feet; we deviated about 500 (3000 MSL) in total.  After the 'clear of conflict' alert we returned to 2500 feet.  We then asked for a new clearance to get back in sequence for arrival.  I informed Potomac that I would file a report. I saw 400 feet on the TCAS as the closest distance between the two aircraft.
[Report narrative contained no additional information]
A320 Captain reported experiencing an NMAC with a helicopter on the Mount Vernon Approach to DCA.