Coffman, R-Aurora, led Romanoff by a margin of 53 to 42 percent at 2 a.m.
During a speech to his supporters, Coffman thanked his supporters while also pledging to listen to the concerns of voters who cast ballots for Romanoff.
"To all of the voters of the 6th Congressional District, I will continue to work hard for you in Washington, D.C.," Coffman promised.
Romanoff, who launched his campaign for Coffman's seat in early 2013 and raised close to $5 million, might have again picked the wrong race at the wrong time.
After passing on challenging Coffman in 2012, a presidential year when high Democratic turnout propelled a no-name challenger to within a couple of points of Coffman in the newly redrawn 6th District, Romanoff chose a 2014 election cycle that became increasingly hostile to Democratic candidates nationwide.
"This has been a very tough race and I am a better candidate for it," Coffman said.
Coffman, a former Marine, has again demonstrated his toughness on the campaign trail, deftly reversing course last year on his immigration stance in order to appeal a new constituency of Hispanic voters and spending countless hours learning Spanish and visiting with his district's other immigrant communities.
Romanoff, a former statehouse Speaker from Denver who unsuccessfully challenged Democratic Sen. Michael Bennet in a costly 2010 primary, ran as a moderate centrist -- his first TV ad outlined a plan to balance the budget -- but might have had a hard time appealing to voters in blue-collar Aurora.