Congress candidate Pratibha Singh was leading with a margin of over 100,000 votes out of around 400,000 votes counted before noon in the Mandi parliamentary bypoll, election officials said in Mandi on Sunday.
The main contest is between Pratibha Singh, wife of chief minister Virbhadra Singh, and Bharatiya Janata Party (BJP) legislator and former cabinet minister Jai Ram Thakur.
Counting of votes began at 8 am and the result is expected by noon, the election official said.
There are two independents also in the fray.
Nearly 53% of the 11,24,786 electors exercised their franchise in two phase of voting held June 23 and 27.
The bypoll was necessitated after Virbhadra Singh relinquished the seat following his election to the state assembly in December 2012.
The Mandi constituency, which includes Kullu, Mandi and some areas of Chamba and Shimla districts, besides the tribal-dominated Kinnaur and Lahaul and Spiti, is one of the biggest in the country.