Colorado Rep. Cory Gardner unseated one-term incumbent Democrat Mark Udall Tuesday to bring yet another crucial Senate seat under Republican control in one of the tightest and most heated races of the 2014 midterm elections. The win saw Republicans capture five of the six seats they need to gain control of the Senate.

Early voting showed Republicans out in force for Gardner, turning in 104,000 more ballots than Democrats in Colorado’s first entirely mail-in election. Gardner led Udall by less than 3 percentage points in each of the last four polls taken in the last week, within the polling margin of error. FiveThirtyEight gave Gardner a 72 percent chance of winning the race just prior to Election Day.

Republicans need to unseat six of the 21 Democratic senators up for re-election to win control of the Senate. Republicans are expected to easily keep control of the House of Representatives.

A key component of Gardner’s campaign was linking Udall to President Barack Obama by highlighting Udall’s loyal record of voting with the president. Udall is considered a “rank-and-file” Democrat by GovTrack. Udall meanwhile, highlighted Gardner’s support for what critics call “personhood” initiatives, a blanket term for legislation that would ban not just all abortions but some contraceptives.

Gardner, 40, served two terms in the U.S. House of Representatives from 2010 until 2014 and was a state representative for six years previously. He deflected much of the criticism Udall sent his way over women’s issues and turned it against Udall by accusing him of running a one-issue campaign.

Udall, the senior U.S. senator from Colorado, was elected with 52 percent of the vote in 2008 over Republican Bob Schaffer after long-serving GOP incumbent Wayne Allard bowed out. Prior to becoming a senator, Udall represented Colorado’s 2nd Congressional District.

As in most other Senate races this year, Gardner’s Republicans outspent Udall’s Democrats. Udall spent about $1 million more than Gardner individually, but Republican interest groups spent nearly $30 million supporting Gardner, compared to their Democratic counterparts, who spent more than $20 million. More money was spent on media advertising in Denver for the race than in any other single market in the country, according to Smart Media Group.