Former astronaut Mark Kelly sworn in as Arizona's newest US senator

Kelly will fill the remainder of the late Sen. John McCain's term.

Mark Kelly was sworn in as Arizona's second Democratic senator Wednesday afternoon, shaving the Republican's majority by one vote for the final weeks of the lame-duck session and marking the first time the state has had two Democrats in the Senate in 67 years.

On the Senate floor, Kelly was escorted to the dais by fellow Arizona Democrat Sen. Kyrsten Sinema where Vice President Mike Pence, as Senate president, delivered the oath of office. All three kept their masks on for the entirety of the swearing-in as Kelly stood six feet back from the vice president with his hand on a Bible.

When the vice president said, "Congratulations," Kelly was met with applause from members on both sides of the aisle. There were roughly a dozen Republicans in the chamber at the time, including Senate Majority Leader Mitch McConnell, who shared an elbow bump with Kelly, as did Pence.

Former Rep. Gabby Giffords, Kelly's wife, watched from seating above the chamber. After the official swearing-in, Kelly moved to the Old Senate Chamber for a photo op.

"This is an honor for me," Pence told him there, according to a pool reporter present. Giffords also let out a brief, celebratory "woo!" after the ceremonial swearing-in.

Kelly defeated GOP Sen. Martha McSally by more the 78,000 votes in a special election to serve out the remainder of the late Sen. John McCain's term which runs until Jan. 2023. (Sally was appointed to the seat in 2018 after Sen. John Kyl, also appointed by Republican Gov. Doug Ducey to fill the post, resigned.)

A Navy veteran and former astronaut, Kelly became a strong advocate for gun control in the aftermath of a failed assassination attempt on his wife. He is one of two Democrats to flip Senate seats this year, joining Gov. John Hickenlooper who beat the Republican incumbent in Colorado, but has the unique position of being sworn in ahead of the new Congress in January since he won a special election.

Kelly's victory helped narrow the Senate GOP's majority from 53-47 to 52-48. The balance of power -- and the promise of Biden passing big ticket legislation he's hoping for -- now hangs on two runoff elections in Georgia. If Democrats win both races, the Senate would be split 50-50, and incoming Vice President-elect Kamala Harris would cast tie-breaking votes in the upper chamber.

Ahead of the ceremony, Kelly and his family paid their respects to McCain at the U.S. Naval Academy Cemetery in Annapolis, Maryland, on Tuesday -- a stark contrast from the attacks of the late senator President Donald Trump has dished out since before taking office.

"Senator McCain has been a hero of mine since I was a young pilot. He left a legacy of service to Arizona and country that can't be matched, but that we should all strive towards," he posted to social media.

After Trump's attacks, Kelly has come to McCain's defense.

He pitched himself as an independent-minded candidate, who would work across the aisle in the Senate to get things done for Arizonans, and also campaigned on a stronger response to the coronavirus pandemic as Trump and allies like McSally downplayed its impact.

"The legacy of this Arizona Senate seat, once held by Senator McCain, is one of independence," Kelly said in a statement following his swearing in. "I am committed to working with Republicans and Democrats and using science, data and facts to craft policies that will help us overcome our greatest challenges, including tackling this virus and getting our economy back on track."

Against the wishes of the president, Ducey signed off on the certification of the state's vote on Monday, telling reporters it will ensure Kelly is sworn into office "as swiftly as possible," and despite Trump's unsubstantiated claims of voter fraud in Arizona, McSally conceded to Kelly on Nov. 13.

It marks a new milestone in an already impressive career.

He served as a naval aviator and flew on dozens of missions, including some during Operation Desert Storm, and in 1996 he and his twin brother Scott were selected to be NASA shuttle pilots. During his five years with the agency, Kelly was a part of four missions to the International Space Station and totaled over 54 combined days in space.

Kelly retired from NASA shortly after Giffords was shot and the couple created a nonprofit gun control group, Americans for Responsible Solutions, which pushed states and congress for more common sense gun laws.

"I learned a lot from being an astronaut. I learned a lot from being a pilot in the Navy, " Kelly said in his campaign announcement video. "But what I learned from my wife is how you use policy to improve people's lives."

ABC News' Trish Turner, Allie Pecorin, Meg Cunningham and Allie Yang contributed to this report.