THE ACFT DEPD GLASGOW, MONTANA, WHERE THE PASSENGER (SECRETARY OF STATE FOR MONTANA) HAD BEEN CAMPAIGNING. INITIALLY, A VFR FLT PLAN HAD BEEN FILED; HOWEVER, DRG ARRIVAL AT HELENA, AN IFR FLT PLAN WAS FILED WHEN THE PLT REALIZED THE WX HAD DETERIORATED. THE FCST HAD INDICATED CHC OF CEILING 2500 BKN WITH LIGHT SNOW. THE PLT WAS CLRD FOR AN ILS RWY 27 APCH AT 2328 MDT. THE PLT ACKNOWLEDGED THE CLNC, BUT A SHORT TIME LATER, RADIO CTC WAS LOST. THE ACFT CRASHED ALONG THE ILS COURSE AFTER IT HAD PASSED THE FINAL APCH FIX/OUTER MARKER. WITNESSES RPRTD THAT A SNOW STORM BEGAN JUST BEFORE THE ACDNT OCCURRED AND THE VISIBILITY WAS NEAR ZERO DUE TO BLOWING SNOW. THE ACFT IMPACTED IN A LEFT WING LOW, NOSE LOW ATTITUDE. WITNESSES SAID THEY HEARD THE ACFT OPERATING AT HIGH RPM AND LOW ALTITUDE BEFORE THE CRASH, OR THAT THE ENG SOUND INCREASED BEFORE IMPACT. ONE WITNESS SAID THE ENG SOUND VARIED FM HIGH TO LOW RPM. NO PREIMPACT PART FAILURE OR MALFUNCTION WAS FND. THE PLT HAD LOGGED 2 HRS IN C-310 AND 3 INSTRUMENT HRS IN PREV 90 DAYS.