Former Democratic Gov. John Hickenlooper defeated Republican Sen. Cory Gardner in the race for the Senate on Nov. 3.
The loss briefly handed Democrats a crucial Senate seat before Republicans flipped a seat of their own in Alabama, where incumbent Sen. Doug Jones (D) lost to Republican challenger Tommy Tuberville. In Kentucky, Senate Majority Leader Mitch McConnell fended off Democrat Amy McGrath.
Republicans currently control the Senate with a 53-47 majority. More seats are on the line across the map.
The Senate majority will be crucial to the winner of the presidential race. The chamber confirms executive branch nominees and can propel the White House's legislative agenda.
Sen. Lindsey Graham (R-S.C.) fended off a tough challenge by Democrat Jamie Harrison, who drew more than $100 million in small-scale donations.
The Senate will welcome some newcomers as others retire. In Tennessee, Republican Bill Hagerty won the seat held by Sen. Lamar Alexander, who is retiring. Republican Cynthia Lummis, the former congresswoman from Wyoming, won the Senate seat opened by retiring GOP Sen. Mike Enzi.
Several Democrats were reelected, including Sen. Dick Durbin of Illinois and Sen. Ed Markey, who survived a primary challenge in Massachusetts. Sen. Chris Coons kept the Delaware seat once held by Biden.
Among Republicans, Tom Cotton in Arkansas, Ben Sasse in Nebraska, Shelley Moore Capito in West Virginia, and James Inhofe in Oklahoma won.
Arizona could see two Democratic senators for the first time since last century if former astronaut Mark Kelly maintains his advantage over GOP Sen. Martha McSally for the seat once held by the late Republican John McCain.
The biggest risks to Democrats were in Alabama and Michigan.
Republicans were expecting to reclaim the seat in Alabama, where Democrat Doug Jones pulled off a rare 2017 special election win in the Trump stronghold.