The race in Colorado's Seventh Congressional District could be decided Friday.
Republican Bob Beauprez's slim 122 vote lead over Democrat Mike Feeley forced a recount. Officials are going over 170,000 ballots.
Jefferson County officials started their recount work Wednesday, joining Adams and Arapahoe county officials who had begun their recounts the day before.
The process is governed by complex election rules and clerks have been working to ensure that the recounts are conducted to the letter of the law under the careful watch of election observers.