ACRID FUMES IN COCKPIT CAUSE ACR FLC TO DECLARE EMER AND LAND AT NEAREST SUITABLE FIELD.

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AT APPROX XA26Z AT FL310; 40-80 NM NNE OF CAE; BOTH THE FO AND MYSELF SMELLED FUMES. WE IMMEDIATELY CALLED THE FLT ATTENDANT AND ASKED WHETHER SHE SMELLED ANY FUMES IN THE CABIN. SHE DID NOT. THE FUMES SMELLED LIKE PROPANE GAS WHICH QUICKLY BECAME STRONGER AND BECAME AN ACRID SMELL. WE IMMEDIATELY DONNED OUR OXYGEN MASKS AND ESTABLISHED COM. I DECLARED AN EMER WITH ATC; REQUESTED A LOWER ALT AND VECTORS TO THE CLOSEST ARPT. AS WE DSNDED; THE FUMES BECAME STRONGER AND MORE ACRID. I DEPLOYED THE PAX OXYGEN MASKS FOR THEIR SAFETY. THE FLT ATTENDANT WAS ADVISED TO HAVE THE PAX USE THE MASKS UNTIL WE LAND. WE LANDED ON RWY 11 AT COLUMBIA; SC; AND CAME TO A STOP ON A HIGH SPD TXWY. I PERFORMED THE MEMORY ITEMS FOR 'EVAC BECAUSE OF CRASH; SEVERE ACFT DAMAGE OR INTERNAL FIRE' BUT ELECTED FOR AN ORDERLY DEPLANING OF THE PAX THROUGH THE FORWARD PAX DOOR. I SEARCHED THE CABIN AND THEN EXITED THE ACFT. THE TIMES STATED BELOW WERE PROVIDED BY CAE CFR WHOSE SOURCE WAS LCL ATC: EMER DECLARED AT XA20Z. LANDED AT XA41Z. THE CFR CONFIRMED THE STRONG PRESENCE OF FUMES IN THE COCKPIT BUT COULD NOT DETERMINE ITS SOURCE. THE NEXT DAY THE FAA AND MAINT DISCOVERED AN 8 INCH CRACK ON THE FO'S R WINDSHIELD; WHICH THEY BELIEVE ALLOWED MOISTURE TO CONTACT THE HEATING ELEMENT AND ARC WITH THE HEATING BLOCK TO CREATE THE ODOR (FUMES) WE SMELLED.