Democrat Mark Kelly projected to flip GOP-held Senate seat

Former astronaut Mark Kelly, a Democrat, will fill John McCain's Senate seat through 2022, ABC News projects.

The race has become one of the most closely watched in part because it's one of the Democrats' best opportunities to flip the Republican-controlled Senate.

If Joe Biden loses the election, Democrats would need to win four Senate seats to gain the majority. If Biden is elected, they would only need three seats as Kamala Harris could break ties in the senate.

Arizona has been counting ballots for two weeks, and early ballots account for 85% of the vote there.

One factor at play is the state's changing demographics. Arizona has one of the fastest-growing Latino populations in the nation, with that community representing more than 30% of the population there, according to Pew Research.

Democrats also showed they were able to win significant votes from independents, which is how a third of voters identify as in the state.

After Sen. John McCain's death in 2018, his appointed successor Jon Kyl stepped in. Kyl retired at the end of that year and Arizona Gov. Doug Ducey appointed McSally to fill the seat.

Two years remain in the term and McSally is running against Kelly to keep her role.

McSally ran for Senate in 2018 for the seat Sen. Jeff Flake vacated with his retirement. She lost to Sen. Kyrsten Sinema, a Democrat.

During McSally's tenure, she has been an outspoken supporter of President Donald Trump, voting to acquit him of articles of impeachment on charges of abuse of power and obstruction of Congress earlier this year. More recently, she has praised the president's handling of the coronavirus.

In March, McSally revealed at a congressional hearing that a superior Air Force officer raped her.

"I also am a military sexual assault survivor, but unlike so many brave survivors, I didn't report being sexually assaulted," McSally said. "Like so many women and men, I didn't trust the system at the time. I blamed myself. I was ashamed and confused. I thought I was strong but felt powerless. The perpetrators abused their position of power in profound ways."

"We are appalled and deeply sorry for what Senator McSally experienced and we stand behind her and all victims of sexual assault," an Air Force spokesperson said. "We are steadfast in our commitment to eliminate this reprehensible behavior and breach of trust in our ranks."

Mark Kelly is a former Navy captain and retired astronaut, who has become a strong advocate for gun control in the aftermath of a failed assassination attempt on his wife, former Rep. Gabby Giffords.

Kelly pitched himself as an independent-minded candidate, who would work across the aisle in the Senate to get things done for Arizonans.

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.

He retired from NASA shortly after his wife 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.

ABC News' Ivan Pereira contributed to this report.