CNN  — 

Democrats picked up a Senate seat on Wednesday when former astronaut Mark Kelly was sworn in as a US senator for Arizona after defeating Republican Sen. Martha McSally last month.

While other senators-elect will have to wait until January to be sworn in for the new Congress, Kelly was able to take the oath of office right away since he won a special election.

A Kelly aide told CNN earlier in the day that Kelly would be sworn in with his wife, former Democratic Rep. Gabby Giffords, and his two daughters by his side, and that his twin brother, Scott Kelly, and family friends would also be attending.

Party control of the Senate still has yet to be determined and now hinges on two Georgia runoffs set for January. Securing the seat from Arizona is nevertheless a major victory for Democrats, who had set their sights on it as one of their early top pickup opportunities in 2020.

The special election victory marks a moment of triumph for Kelly, a retired Navy captain and NASA astronaut, that comes in the aftermath of tragedy.

Kelly was thrust into the national spotlight in 2011 when his wife, Arizona’s then-US Rep. Gabby Giffords, was shot in the head and nearly killed, an event that sent shock waves throughout the nation.

He later turned into a political activist, launching a group called Americans for Responsible Solutions alongside his wife and fighting for gun control policies like universal background checks and so-called red flag 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.”

His entry into politics has marked the start of a new chapter in an already esteemed career.

Kelly flew into space four times, twice as commander of a space shuttle. He and his identical twin, Scott Kelly, who is also an astronaut, participated in a one-of-a-kind “twin study” to see what spaceflight does to the human body.

Mark Kelly is also a retired Navy captain and pilot who flew 39 combat missions in the first Gulf War.

After prevailing over McSally in the special election, he will serve out the remaining two years of the late Sen. John McCain’s term. He will be able to run for reelection to serve a full term in 2022.

McSally, the Air Force’s first female pilot to fly in combat, was appointed to fill the seat after she narrowly lost a race to Sinema for Arizona’s other Senate seat in 2018.

Sending two Democrats to Washington will mark a first in nearly 70 years for Arizona.

Democrats have made inroads in the state long considered a conservative stronghold but which has increasingly become a highly contested battleground. President-elect Joe Biden won the state last month in another major victory for the Democratic Party.

Mark Kelly, who will be sworn in as a US senator on Wednesday, and his wife, former Rep. Gabby Giffords, visit the gravesite of the late Sen. John McCain in Annapolis, Maryland, on Tuesday.

Kelly has said that serving the remainder of McCain’s term is deeply significant to him, describing the late senator as someone he looked up to when he was a 22-year-old pilot in flight school.

On Tuesday, Kelly and Giffords visited McCain’s gravesite in Annapolis to pay their respects before he is sworn in today.

“Senator McCain has been a hero of mine since I was a young pilot,” Kelly tweeted, “He left a legacy of service to Arizona and country that can’t be matched, but that we should all strive towards.”

Kelly has recently spoken to Cindy McCain and expressed his desire to use McCain’s desk in his Senate office.

This story has been updated with additional developments Wednesday.

CNN’s Ali Zaslav contributed to this report.