NMAC REDUCED TO A POTENTIAL CONFLICT BY THE TCASII TA RA ON ACR MLG.

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WE WERE ON A BASE LEG VECTOR FOR THE ILS 22L APCH TO EWR. WE WERE IN IMC AT 3000 FT MSL. IN AND OUT OF THE RAGGED BASES OF THE OVCST; ON AN ASSIGNED HDG; 210 KIAS WITH SLATS EXTENDED AND FLAPS AT 11 DEGS. OUR TCASII SHOWED NUMEROUS TARGETS BUT DID NOT HAVE OUR CLOSE ATTN DUE TO THE WORKLOAD AT THIS PHASE OF FLT UNTIL WE RECEIVED THE 'TA.' THE TARGET WAS AT 11 O'CLOCK AND 400 FT ABOVE OUR ALT WITH A RANGE OF 2 1/2 MI; CONVERGING WITH OUR FLT PATH. THE TA QUICKLY BECAME A 'DSND' RA TO WHICH I RESPONDED WITH AN IMMEDIATE DSCNT AT 1200 FPM. AS WE REACHED 2600 FT MSL THE RA REVERTED TO 'MONITOR VERT SPD' ALLOWING US TO LEVEL OFF. WE ADVISED ATC OF THE DEV IN RESPONSE TO A TCASII RA. HE RECOMMENDED WE RETURN TO 3000 FT AS SOON AS POSSIBLE AND THAT HE HAD 'UNCTLED' TFC ABOVE US. WE CLBED BACK TO 3000 FT UPON RECEIVING THE 'CLR OF CONFLICT' ADVISORY; NEVER GAINING VISUAL CONTACT WITH THE TFC. ITS FLT PATH ON OUR TCASII SCOPE WENT NEARLY OVERHEAD MAINTAINING ITS ALT (APPROX 3400 FT MSL). TFC WOULD HAVE TO HAVE BEEN IN IMC IN THE TCA AT 3400 FT MSL SO I DO NOT UNDERSTAND THE 'UNCTLED' NATURE OF ITS FLT PATH. CTLR WORKLOAD WAS FAIRLY HIGH AT THE TIME; BUT I DID NOT NOTICE ANY INSTRUCTIONS ISSUED BY THE CTLR THAT WOULD HAVE CORRESPONDED WITH SUCH TFC; SO I FULLY BELIEVE HE WAS NOT IN CONTACT WITH THAT ACFT.