NMAC ON SHORT FINAL.

Narrative

OUR ACFT WAS CLRED FOR A VISUAL APCH TO DCA RWY 21 TO HOLD SHORT OF RWY 18. WHILE ON BASE TO FINAL LEG; TWR TOLD KING AIR 'Y' THAT HE WAS #2 FOR RWY 21. 'TFC WAS ON A L BASE.' 'Y' BEING ON A RIVER VISUAL APCH TO RWY 18 WAS TO CIRCLE TO LAND ON RWY 21; FOLLOWING OUR ACFT. 'Y' RESPONDED THAT HE DID NOT HAVE THAT TFC IN SIGHT! AT THAT TIME WE TURNED BASE TO FINAL ABOUT 1 MI FROM THE END OF RWY 21. A DIFFERENT VOICE IN DCA TWR CAME ON AND TOLD KING AIR TO GAR AND HE DID. AS OUR ACFT ROLLED WINGS LEVEL AT ABOUT 400 FT AGL WE SAW KING AIR APCHING OUR ACFT RAPIDLY FROM THE R SIDE; APPARENTLY AT OUR ALT. I PUSHED OUR ACFT'S NOSE DOWN TO MAINTAIN A COMFORTABLE SEPARATION AND FOLLOWED THE KING AIR'S FLT PATH CROSS FROM R TO L. TCASII DISPLAYED HIS TARGET AT PWE 200 FT AND NO PERCEPTIBLE HORIZ DEV. ESTIMATED HORIZ DISTANCE FROM 500-1000 FT. OUR ALT AT THIS TIME WAS WELL BELOW THE VASI INDICATING ABOUT 300 FT AGL AND 1/2 MI FROM THE THRESHOLD OF RWY 21. DCA TWR SHOULD NOT BREAK OFF TFC FROM RWY 18 TO CIRCLE TO RWY 21 TO GET OUT DEPS. IF OUR ACFT REMAINED ON THE VASI TO RWY 21 I WOULD NOT BE WRITING THIS NOW.