AN ACR B737 FLC RPTS THAT AN APCH CTLR CLRED THEM FOR A VISUAL APCH IN NON-VISUAL CONDITIONS. THE CTLR HAD NOT ASKED THE FLC IF THEY SAW THE FIELD OR OTHER VISUAL CUES BEFOREHAND. THE FLC REFUSED THE CLRNC.

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ACFT WAS BEING RADAR VECTORED TO FLY THE MT VERNON VISUAL APCH; WHICH IS A VFR MANEUVER. APCH CLRED FLT FOR THE MT VERNON APCH WITHOUT ASKING THE FLT IF IT HAD VISUAL CONTACT WITH THE RIVER IN ORDER TO FLY THE APCH. FLT INFORMED ATC THAT WE COULD NOT MAINTAIN VISUAL; AS WE WERE IN MODERATE RAIN. APCH THEN CLRED US TO 16000 FT MSL AND GAVE US A VECTOR TO INTERCEPT THE LOC TO RWY 36. ATC SHOULD NOT CLR AN ACFT FOR THE MT VERNON VISUAL WITHOUT ASKING THE ACFT IF IT HAS GOT THE VISUAL LANDMARKS IN SIGHT. SUPPLEMENTAL INFO FROM ACN 386310: WE WERE TOLD EARLIER TO EXPECT ILS RWY 36. APCH (AS USUAL FOR DCA) WAS OVERLOADED. IN AN ATTEMPT TO REDUCE SEPARATION (I THINK) HE SUDDENLY STARTED ISSUING VISUAL APCHS. HE NEVER INQUIRED IF WE WERE IN VISUAL CONDITIONS. IT WAS DIFFICULT TO INFORM APCH WE COULD NOT DO VISUAL DUE TO HIS 'RAPID FIRE' NON-STOP INSTRUCTIONS. THIS TYPE OF OVERLOADING AND REDUCING BELOW NORMAL SEPARATION IS BECOMING THE NORM AT DCA. SOMEONE NEEDS TO CORRECT THIS BEFORE AN ACCIDENT OCCURS.