PHOENIX (AP) — Democrat Mark Kelly opened a big lead Tuesday against Republican Sen. Martha McSally in early election results for an Arizona race that will be crucial in determining control of the U.S. Senate.

Democrats are optimistic that Arizona's changing demographics and President Donald Trump's unpopularity among some suburban voters can push Kelly, a retired astronaut, to victory in Tuesday's election. He is the husband of former U.S. Rep. Gabrielle Giffords, who was injured in an assassination attempt in Tucson in 2011.

Kelly led 55% to 45% with 75 percent of the expected votes counted. The tally includes early votes cast through the weekend, and the race will likely tighten as officials tally Election Day votes, which are expected to favor Republicans.

Kelly all but declared victory, saying: “I’m confident that when all the votes are counted, we’re going to be successful in this mission.”

“The work starts now. And we desperately need Washington to work for Arizona,” Kelly told a small group of family and reporters in Tucson. “My top priority is making sure we have a plan to slow the spread of this virus, and then getting Arizona the resources our state needs right now.”

After his speech, Kelly clasped arms with Giffords and his two adult daughters as a massive screen behind him showed video feeds of supporters cheering from their living rooms.

McSally was not expected to speak Tuesday night. She expressed confidence Monday to supporters that she could win.

No matter who wins, Arizona will have a senator from Tucson for the first time since Democrat Dennis DeConcini left office in 1995.

An influx of new voters in the fast-growing suburbs of Phoenix and extensive get-out-the-vote effort in the Latino communities in Phoenix and Tucson helped put Arizona, a longtime Republican stronghold, in play for Democrats. The trend accelerated with a shift away from the GOP among white suburban women who turned against Trump.

The 2018 victory of Sen. Kyrsten Sinema, the first Democrat to win an Arizona Senate seat in 30 years, over McSally illustrated the changing nature of the state.

After her defeat, Republican Gov. Doug Ducey appointed McSally to McCain’s former seat in 2018.

McSally was a trailblazing woman in the U.S. Air Force — the first woman to fly in combat and to lead a fighter squadron. More recently, she revealed a darker side of her military career, disclosing last year that she was raped by a superior officer.

Kelly flew combat missions for the Navy during Operation Desert Storm before becoming a test pilot and later an astronaut. He flew four missions to the International Space Station. After Giffords was shot, the couple founded a group that works to elect lawmakers who support gun-control.

Kelly had a big fundraising advantage, outraising McSally by more than $30 million through Oct. 14. Both candidates shattered fundraising records for Arizona, raising more than $146 million between them, more than triple the combined spending on the 2018 McSally-Sinema race. Independent groups spent tens of millions more to sway voters.

That money flooded airwaves, websites and mailboxes with ads.

Democrats portrayed Kelly as an independent thinker not beholden to either party. They slammed McSally’s votes to repeal the federal health care law, which they said would make it expensive, if not impossible, for people with pre-existing conditions to get health care coverage.

Kelly tried to tie McSally to President Donald Trump and his handling of the coronavirus.

Republicans presented McSally as a tough-minded fighter on behalf of Arizona residents. They targeted Kelly’s business ties to China, pointing to an investment from a Chinese firm in a business Kelly co-founded and alleging he would “do anything for a buck and say anything for a vote.”

McSally also tried to deflate Kelly's independent image by linking him to left-wing members of his party, such as Vermont Sen. Bernie Sanders and Minnesota Rep. Ilhan Omar. She warned that a Kelly victory could give Democrats control of the Senate and usher in liberal priorities such as Medicare for all and the Democratic Green New Deal climate plan, though Kelly has said he opposes both.

Tuesday’s winner will take office as early as Nov. 30, finish the last two years of McCain’s term and then face re-election in 2022.

A Kelly victory would give Democrats control of both of Arizona's Senate seats for the first time in nearly 70 years.

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Find AP’s full election coverage at APNews.com/Election2020.