B727 CREW TAKES EVASIVE ACTION AFTER RECEIVING TCASII RA IN BWI AIRSPACE.

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WE WERE CLRED TO DSND TO 4000 FT ON VECTORS FOR A VISUAL APCH TO RWY 33L AT BWI. PASSING 6000 FT; APCH CTL REQUESTED THAT WE EXPEDITE OUR DSCNT; THEN A MOMENT LATER ADVISED US OF 12 O'CLOCK TFC; OPPOSITE DIRECTION AT 4500 FT. WE RECEIVED A TA WHEN WE WERE APCHING 5000 FT; AND THE TARGET WAS DISPLAYED AT 4 MI. WE SLOWED OUR DSCNT; ANTICIPATING A TFC CONFLICT; AND ADVISED ATC THAT WE WOULD 'LEVEL AT 5000 FT' DUE TO IMPENDING CONFLICT (POOR CHOICE OF WORDS!). THE CTLR ASKED US IF WE HAD AN RA AND WE REPLIED NO BUT WERE EXPECTING ONE; AND IN FACT WE HAD AN RA 5 SECONDS LATER. WE COMPLIED WITH THE TCASII RA 'CLB' COMMAND; ADVISED THE CTLR; AND WE RECLRED TO MAINTAIN 5000 FT. THE CTLR WAS ANGRY AND STATED THAT HE CONSIDERED WHAT I HAD INTENDED TO BE A MOMENTARY LEVELOFF AS AN ALTDEV BECAUSE WE DID NOT CONTINUE DSNDING TO 4000 FT WITH ONLY A TCASII TA. I HAD CONSIDERED CONTINUING THE DSCNT BEFORE THE RA BUT IT WAS OBVIOUS THAT AN RA WAS IMMINENT. CLRER COMS WITH THE CTLR AND MORE AGGRESSIVE COMPLIANCE WITH ATC INSTRUCTIONS MIGHT HAVE PREVENTED THIS MISUNDERSTANDING. IT IS UNFORTUNATE THAT WITH A CONFLICT SO OBVIOUS AND IMMINENT; THE ONLY 'LEGAL' COURSE OF ACTION IS TO CONTINUE TOWARD DISASTER AND BEGIN EVASIVE MANEUVERS WITH THE LESS TIMELY (IE; LESS SAFETY MARGIN) RA.