A C172 SINGLE PLT ON THE LOC APCH FOR RWY 15 AT MTN WAS ALERTED BY HIS SAFETY PLT THAT HE WAS TOO LOW.

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WE WERE ON THE LOC 15 APCH TO MTN WITH AN 800 FT CEILING. POTOMAC APCH CLRED US FOR THE APCH; BUT DID NOT HAND US OFF TO MTN TWR. I BECAME DISTR TRYING TO RAISE POTOMAC ON THE RADIO AND THEN TRYING TO CALL MTN TWR DIRECTLY. I FOCUSED ON THE RADIO AND FORGOT TO MONITOR MY ALT. MY SAFETY PLT FINALLY POINTED OUT THAT I WAS AT 400 FT; WHICH IS ABOUT 150 FT BELOW MDA; AND WAS DSNDING RAPIDLY. FORTUNATELY; WE HAD BROKEN OUT OF THE CLOUDS AND HAD THE FIELD IN SIGHT SO I IMMEDIATELY CLBED TO A SAFE ALT AND COMPLETED THE APCH AND LNDG. IF THERE HADN'T BEEN A SECOND PLT ABOARD TO MONITOR THE APCH; I MIGHT HAVE FLOWN A PERFECTLY GOOD AIRPLANE RIGHT INTO THE GND! I FORGOT A FUNDAMENTAL RULE OF INST FLYING: FIRST YOU AVIATE (YOU FLY THE AIRPLANE UNDER CTL); THEN YOU NAV (THAT IS; KEEP THE AIRPLANE IN THE RIGHT POS AND ALT). ONLY THEN DO YOU TRY TO COM. I ALLOWED MYSELF TO GET DISTR OVER SOMETHING THAT WAS NOT IMPORTANT. I SHOULD HAVE EITHER CONTINUED THE APCH; TREATING IT AS A 'LOST COM' SIT; OR BETTER YET; ASKED THE SECOND PLT TO TRY TO STRAIGHTEN OUT THE HDOF WITH APCH. NOT ONLY DID I BECOME DISTR; BUT I ALSO FAILED TO USE ALL THE RESOURCES AT MY DISPOSAL. I HOPE I LEARNED A LESSON TODAY BECAUSE I'M LUCKY TO BE ALIVE!