Rep. Cory GardnerCory Scott GardnerSessions floats federal law that would protect states that decriminalize marijuana RNC mum on whether it will support Trump-backed Corey Stewart Overnight Health Care — Sponsored by PCMA — Dems see midterm advantage in new ObamaCare fight MORE (R-Colo.) orchestrated a stunning defeat of Democratic Sen. Mark UdallMark Emery UdallSenate GOP rejects Trump’s call to go big on gun legislation Democratic primary could upend bid for Colorado seat Picking 2018 candidates pits McConnell vs. GOP groups MORE (D-Colo.), delivering a serious blow to Democrats’ hopes of holding onto the Senate.

Gardner took 51 percent support to Udall’s 44 percent when The Associated Press called the race, with 65 percent reporting.

He said in his victory speech that his win was a warning not to either party, but to anyone "who failed to act on our nation’s greatest challenges."

"It is time for a new direction, it is time for a new way forward," Gardner said, echoing his campaign slogan.

The GOP win is sure to ripple beyond Colorado, as it raises questions about the vaunted Democratic ground game and the efficacy of one of the party’s main arguments against Republicans — the argument that the GOP is waging a “War on Women.”

That was Udall’s primary attack on Gardner, alleging he was on the wrong side of Colorado’s women because of his previous support for a state measure that would’ve effectively outlawed abortion and restricted some forms of birth control in the state.

More than half of his ads hammered Gardner on the issue, and Democrats insisted it was a winning argument, pointing to the success of Sen. Michael BennetMichael Farrand BennetOvernight Finance: Trump wants Russia back in G-7 | Senators, allies push back | House approves first fiscal 2019 spending bills | Dems want insider trading probe over job tweet Dems want insider trading probe after Trump jobs report tweet Clinton on his reading habits: 'I like a fast ride' MORE (D-Colo.) running on a similar strategy in 2010.

But this time around, the strategy brought Udall widespread ridicule, with the Denver Post accusing him of running an “obnoxious one-issue campaign” in its endorsement of Gardner, and even his own supporters expressing public frustration with the campaign’s focus.

Gardner entered the race to much GOP fanfare and quickly cleared the field, putting a once-safe seat into play early on.

Once rated the 10th most conservative member of the House, the congressman moderated his stance on reproductive rights by disavowing his support for the state measure in question early on in the race. Democrats still pointed to his co-sponsorship of the federal version of the bill in their attacks, but Gardner largely neutralized those attacks with his early shift.

He also became the first Republican to call for over-the-counter birth control to be offered without a prescription, which helped boost his moderate credentials. And he hit back on Udall by tying him to Obama at every turn, who suffered lower approval ratings in Colorado than he did nationwide.

Public polling has shown Gardner opening up a solid lead on Udall for the past month and a half, but Udall’s internal polls showed the race tied, and Democrats insisted their strong ground game in the state — three times the operation that drove Bennet to a surprise win in 2010 — would help them make up the difference on Election Day.

Gardner's win suggests Democrats may have underestimated the investment, and the gains, Republicans made in their own ground game.