By Daniel Wallis and Keith Coffman
DENVER (Reuters) - Republican congressman Cory Gardner defeated the Democratic incumbent in Colorado's U.S. Senate race on Tuesday, handing Republicans a big win in a swing state where the Democratic governor was also narrowly trailing his challenger.
Senator Mark Udall's re-election had looked fairly safe until February, when Gardner, 40, said he was leaving a secure seat in the U.S. House of Representatives to take on the first-term senator.
"We won this election because we asked Coloradans ... to look ahead to a future that is brighter," Gardner told jubilant supporters at Republican campaign headquarters.
"We can fix this nation's problems together. ... What is happening in Washington isn't working and it has to stop."
Democrats said their Republican rivals made Colorado "Ground Zero" in their battle for the U.S. Senate, and the 64-year-old Udall was seen as vulnerable because of his support for the Affordable Care Act, or Obamacare, which was highly unpopular in the state.
As Republican supporters cheered when it became clear the party would win control of the U.S. Senate on Tuesday night, fire marshals had to ask some revelers to leave, saying the room at a suburban Denver hotel was too crowded.
With 83 percent of precincts reporting, Gardner had 50.6 percent of the vote to Udall's 44 percent, according to the state's flagship newspaper, the Denver Post. The son of a tractor salesman from a conservative eastern Colorado farming district, Gardner won the newspaper's endorsement last month.
Udall, whose father once sought the Democratic presidential nomination and whose cousin is a Democratic senator from New Mexico, had followed a successful strategy used by Colorado Democrats in the past by putting women's issues front and center.
But his bid to paint Gardner as an extremist on abortion appeared to have backfired, with the Post calling it an "obnoxious one-issue campaign" that insulted voters.
Udall said in a concession speech that serving in the Senate had been "the greatest privilege of my life."
Governor John Hickenlooper, a former Denver mayor and brew-pub magnate who has been spoken of as a possible Democratic vice presidential candidate in 2016, was fighting to fend off a challenge from former Republican U.S. Representative Bob Beauprez.
With 86 percent of precincts reporting, Beauprez led with 48.0 percent of votes to Hickenlooper's 47.5 percent.
Both candidates urged supporters to go home and get some rest as a trickle of results showed the governor narrowing the gap with his rival to just a few thousand votes.
"Do not lose heart. Get a little sleep," Hickenlooper said.
The governor, who ran for a second term on Colorado's strong economic performance under his watch, has faced controversies over a proposed school reform tax increase and stricter gun laws introduced after a movie theater shooting in a Denver suburb and the Sandy Hook school shooting in Connecticut.
Beauprez, who lost a governor's race eight years ago, has also criticized Hickenlooper's backing for a renewable energy mandate, as well as a failed $950 million tax hike for education reform proposed by the state's Democratic-led Legislature.
(Additional reporting by Rick Wilking in Denver, and Anna Yukhananov and Susan Cornwell in Washington; Editing by Doina Chiacu, Eric Walsh, Peter Cooney and Ken Wills)