Congressman Cory Gardner unseated Sen. Mark Udall in Tuesday's election, becoming the first challenger to defeat an incumbent senator in Colorado in 36 years.

WATCH: Mark Udall concedes to Cory Gardner in Senate race

Colorado's decision, one of the most-watched in the nation, helped tip the Republicans' battle for control of the U.S. Senate.

"It finally happened," an ecstatic Gardner told cheering supporters.

Gardner led 51 percent to 44 percent on a night that had long-suffering Colorado Republicans dreaming of a formerly blue-leaning state awash in red.

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"The people of Colorado have had their voice heard," Gardner said. "They are not red. They are not blue. But their message is crystal clear to Washington D.C., 'Get your job done and get the heck out of the way.' "

Party spokesman Owen Loftus called it a "great night for the Colorado GOP."

"The Democrats' reign is over," Loftus said. "Coloradans have cast deciding votes to change the direction of our state."

Udall conceded just after 10 p.m., thanking the people of Colorado and calling "United States senator from Colorado the best job in the world." It was an emotional speech, his voice wavering, as he called the job the "greatest privilege" of his life.

Udall drew laughs when he paraphrased an old line once used by his father, Mo Udall: "The voters have spoken, the blankety-blanks."

Udall thanked voters for the six years he served in the Senate.

"Maybe Thursday or maybe Friday we're all going to go back to work to make sure our communities match our scenery," he said.

The crowd was immense at the Republicans' victory party at the Hyatt in the Denver Tech Center, where cheers erupted from party-goers whenever news of Gardner's lead flashed on television screens.

The Hyatt ballroom was so crowded that security guards stopped people outside from entering. Congressman Mike Coffman's campaign manager, Tyler Sandberg, was able to bring Coffman's family through a side door only after pleading with security guards.

Republicans hadn't won a gubernatorial or U.S. Senate race in Colorado since 2002. In 2010, Democrat Michael Bennet beat Republican opponent Ken Buck by a mere 1.67 percentage points.

Cory Gardner wins Senate seat
Cory Gardner takes the stage with his wife Jamie to make an acceptance speech after winning the U.S. Senate seat during the Colorado GOP's election night event at the Hyatt Denver Tech Center at 7800 East Tufts Ave in Denver, November 4, 2014. (Craig F. Walker, The Denver Post)

At the Democrats' party at the downtown Westin, a collective and loud "ohhh" rippled through the crowd when Gardner was projected as the winner by television networks just after 8 p.m. The party, which had been rocking all evening to Elvis and the Beatles, started to thin. The mood dipped.

An escalator going down, toward the hotel exit, filled with disappointed party-goers.

"It is kind of sad in a way, but in my opinion, it is a cycle," said voter Javier Pineda. "Sometimes, you have good years. Sometimes you don't. This year is definitely getting the bad end of the stick."

Another Udall supporter, Saida Perez, a native of Guatamala who became a U.S. citizen 15 years ago, was disappointed because Udall was "so strong on immigration."

Community activist Jeff Fard said the Democrats' poor showing signaled that more of those who believe in the party's message should get involved. "You have to come out. If you don't, you are going to lose by default. If this doesn't wake them up, I don't think anything will," he said. "The Millennials, they should be here."

Republicans screamed and hugged one another when the first Senate numbers appeared on TV screens, showing Gardner with a lead over Udall.

"I was hoping, but I didn't know what to expect," said Tricia Graf of Centennial.

Lily Nunez, a member of the Republican National Committee, said it was about time Republicans had something to cheer about.

"Has it been decades?" she asked.

Mark Udall
Senator Mark Udall holds back tears as he reacts to the crowd as he gives his concession speech at the Democrat Party at the Westin November 4, 2014 in Denver. (John Leyba, The Denver Post)

Over and over again, Republicans talked about the angst of previous elections — Mitt Romney in 2012, narrowly losing the Senate race in 2010, the blowouts in 2008 and 2006 and the Democratic takeover of the legislature in 2004.

"We live in a democracy. The two parties, they swing back and forth and balance each other out," said former Rep. Glenn Scott. "Tonight, the pendulum is swinging back our way."

Republican voter Tiko Hardy, 40, of Colorado Springs, said she cares about women's issues but voted for Gardner because he is better for the economy.

"Having a right to choose is very important. I don't think anybody's going to overturn Roe v. Wade," she said. "The GOP's been very clear: We want people to have a right to choose, but we don't want to pay for it. I think that's what keeps getting lost."

From Udall's first ad, the campaign focused on women's issues including abortion and the cost of birth control.

Early on, Democrats believed Gardner could not win a statewide race given that Colorado voters twice have easily knocked down "personhood" ballot measures he supported that would have given embryos full rights at the moment of conception.

But, political analysts said, the one-track crusade blasting Gardner's record on personhood in Colorado and in Congress backfired, and voters grew weary of it. Instead, Udall was mocked as "Mark Uterus" and accused of running a campaign that focused on Gardner's record on one issue instead of his own work in Washington.

Obamacare and the battle over ballot measures to limit fracking by oil and gas companies were expected to drive the Senate race. But the health care law fell off the ad-play list, in part, Democrats say, because the new program showed early signs of success. The fracking measures were pulled after marathon behind-the-scenes negotiations engineered by Democratic Gov. John Hickenlooper, who feared they would destroy the state's business climate.

When Gardner announced in March he was wrong to have supported the personhood measures, Democrats were incredulous.

"His beliefs haven't changed, but his ambitions have," Udall said.

Udall pointed out that Gardner still supported the federal Life Begins at Conception Act, which critics said would have the same impact as the state proposals.

Gardner wrote an opinion piece in The Denver Post in June proposing to make the birth control pill available over the counter, a move his fans said showed he wasn't interesting in banning birth control but expanding access. Democrats countered that his proposal would make the pill more expensive.

Udall's first campaign ad, in April, noted Gardner sponsored a bill in the state legislature to make abortion a felony and was on a crusade to outlaw birth control. That Udall didn't begin with the obligatory "feel good" ad about himself surprised political consultants.

Democratic strategist Laura Chapin seethed as events unfolded, including the over-the-counter proposal. She said Gardner has been allowed to get away with lies.

"Gardner has been unable to answer simple questions about where he stands," she said. "He's run the most cynically dishonest campaign I've seen in 20 years in politics."

But Republican strategist Katy Atkinson said winning campaigns do two things: They define their candidate, and they define their opponent.

"The strategy appeared to be 'Gardner's so awful that anyone is better, so vote for me,' " she said.

With control of the Senate at stake, the national parties and special-interest groups have dumped $69.4 million into the Udall-Gardner battle — the second most in the country behind North Carolina, according to the Center for Responsive Politics.

So many journalists, including national and international, were in Colorado for Election Day that Udall spokesman Chris Harris joked, "It's like the Olympics."

It seemed like a pipe dream back in February when Gardner gambled on giving up his safe congressional seat for the chance at ousting Udall.

"I don't know what polls Cory Gardner is reading, because he's not going to win," Udall said at the Denver County Democratic Assembly in April.

Gardner was smiling and in high spirits Tuesday even before polls closed, according to his campaign manager.

"He'll never stop smiling. There's been a lot of energy around this race for a while," Chris Hansen said.

"We feel good. This is fun. It's been nasty and cynical at times — it's tough to stare down (U.S. Senate Majority Leader) Harry Reid's hit machine, but we feel we have presented a positive vision for the entire state, and we're proud of the campaign that we've run."

Social media was on fire about the Senate race all Election Day, including tweets calling Gardner a liar regarding his stance on "personhood" and pro-life proposals.

"Who cares? It's Twitter," Hansen said. "A bunch of people living in their parents' basement sitting around and agreeing with each other. I don't care what they say. Cory's great. He's going to be great for the state."

Staff writers Tom McGhee and Laura Keeney contributed to this report.

Cory Gardner (R)50.6 percentMark Udall (D)44.0 percentWith 82 percent reporting