A B737-400 CAPT RPTS REFUSING A CLRNC THAT WOULD HAVE TAKEN THE ACFT INTO A LINE OF TSTMS. AFTER A VERBAL EXCHANGE OVER THE RADIO WITH ATC; THE CREW EVENTUALLY RECEIVED VECTORS AROUND THE SIDE OF THE TSTMS.

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WE DEPARTED RWY 28 AT BWI. THERE WAS A SMALL LINE OF CELLS (VISUAL AND RADAR) TO THE N OF OUR FLT PATH. BWI DEP GAVE US A TURN TO THE N SHORTLY AFTER TKOF. (FO FLYING) I TOLD ATC WE COULD NOT TAKE A TURN TO THE R 'BECAUSE OF WX.' I TOLD HIM WE NEEDED TO GO W APPROX 30- 35 NM TO GET AROUND THE LINE. ATC SAID THAT WAS IMPOSSIBLE AND THAT THE ACFT AHEAD OF US WAS PROCEEDING N AND WAS ONLY ENCOUNTERING RAIN. (WE WERE PROCEEDING W AT 5000 FT). TO OUR N; WE COULD SEE RAPIDLY BUILDING CLOUDS THROUGH THE MID-TEENS TO LOW 20000 FT LEVELS WITH HVY LIGHTNING. ALSO I COULD NOT PAINT A CLR PATH THROUGH THE LINE WITH AIRBORNE RADAR. ATC AGAIN TOLD US TO TURN N. I TOLD ATC THERE WAS 'HVY LIGHTNING' AND THERE WAS NO WAY WE COULD TAKE A TURN N. ATC THEN SAID WE WOULD HAVE TO RETURN TO BWI. ATC GAVE US A CLRNC 'L TURN DIRECT BWI; DSND AND MAINTAIN 3000 FT.' WE STARTED A L TURN AND I QUESTIONED ATC ABOUT THE DSCNT TO 3000 FT. ATC THEN CHANGED THE ALT TO 'MAINTAIN 5000 FT.' WE WERE THEN VECTORED UNEVENTFULLY AROUND THE E SIDE OF THE WX AND ON COURSE TO OUR DEST. IN OVER 30 YRS OF FLYING (GA; MIL; AND AIRLINE); I HAVE NEVER HAD AN ATC CTLR TRY SO HARD TO INTIMIDATE ME INTO FLYING INTO A TSTM. I REALLY THOUGHT FOR A WHILE THAT I WAS GOING TO HAVE TO DECLARE AN EMER TO AVOID ATC'S VECTORS INTO A TSTM.