A B737-500 FLC REQUIRED ASSISTANCE FROM ARTCC TO COMPLY WITH AN ALT XING RESTR IN AN AUTOFLT DSCNT INTO DCA.

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APPROX 50 DME N OF BAL AT FL220; WAS GIVEN CLRNC TO CROSS 20 MI N OF BAL AT 12000 FT. SET UP FMC AND MCP TO MEET XING RESTR. WENT BACK TO TRYING TO FIGURE OUT ATIS FOR DCA; WHICH DISTR ME FROM MONITORING DSCNT. FO WAS PRETTY NEW AND NO HELP AS A BACKUP TO ME. THIS WAS A HUMAN PERFORMANCE MISTAKE BIG TIME! WHEN A HABIT PATTERN IS BROKEN OR A DISTR OCCURS IN THE COCKPIT; YOU CAN'T BE DILIGENT ENOUGH. SUPPLEMENTAL INFO FROM ACN 447911: BOTH THE CAPT AND I BECAME PREOCCUPIED WITH AN UNUSUAL ATIS. ATC CTR CTLR ISSUED US A XING RESTR TO CROSS 20 MI N OF BAL AT 12000 FT. SINCE THE CAPT WAS FLYING HE PROGRAMMED THE FLT COMPUTER TO COMPLY WITH THE RESTR AND I DOUBLECHKED THAT IT WAS ENTERED CORRECTLY AND THAT THE APPROPRIATE ALT WAS ALSO SELECTED ON THE MCP TO ALLOW THE PLANE TO AUTOMATICALLY COMPLY WITH THE XING RESTR. WE BOTH RETURNED OUR ATTN TO THE ATIS. SINCE NEITHER OF US WAS WATCHING FOR THE DSCNT; WE BOTH MISSED THE FACT THE PLANE DIDN'T START DOWN. ATC NOTICED THIS AND GAVE US A VECTOR FOR A 360 DEG TURNING DSCNT.