Republicans have long thought they had a chance to take the majority in the state Senate, where Democrats currently hold a one-vote advantage. But the strength of their results in the House surprised many observers.
With tens of thousands of ballots still uncounted in two key counties, control of the state legislature may not be known until later this week.
The outcome of two Senate races and up to six House elections remain unknown as of midday Wednesday, which could tilt control of the chambers to either party.
Democrats currently have a nine-vote majority in the House. Republican candidates hold narrow leads in five of the six undecided races.
No one is claiming victory yet, since potentially thousands of votes remain uncounted in some of those districts.
"The House never really entered my mind," said former GOP chair Dick Wadhams, who now consults on campaigns. "I thought there was an opportunity to close the gap," but not to take control.
Wadhams credits Republican Senator-elect Cory Gardner's strong showing in helping draw voters to down-ballot races.
Balance of power hinges on obscure race
Adams County could be home to the biggest House upsets. Representatives Jenise May, D-Aurora, and Joe Salazar, D-Thorton, were not considered vulnerable candidates. But as of Wednesday, both trail their challengers by hundreds of votes.
In the Adams-based Senate District 24, which could be crucial to control of that chamber, Republican Beth Martinez Humenick leads Democrat Judy Solano by 1,216 votes.
However, Adams County still had 15,000 uncounted ballots as of midday Wednesday. According to Adams County public information officer Jim Siedlecki, those ballots have to be validated by hand because of the county surveyor's race.
In that contest, a lack of approved candidates led to a write-in-only race. According to Siedlecki, each of those ballots has to be examined by hand as a result.
The county plans to keep staff counting ballots until late Wednesday evening, but doesn't expect to have a final tally until Thursday morning.
"We can't expect them to be focused if they work 18-hour-days," says Siedlecki. "At some point, people need to sleep."
In Jefferson County, where incumbent Democratic Senator Cheri Jahn started out Wednesday with a slim, 31-vote lead over her opponent, Republican Larry Queen, there are still around 14,000 votes left to be counted.
Caroline Carver with the Jefferson County Clerk's office says Colorado's new election law made it hard to predict how many ballots would come in on Election Day, but that 14,000 is a "pretty typical" number to have left over.
Carver says the county hopes to complete its tabulation Thursday morning.
"A lot of people are watching us closely today," says Carver.