Democrat Kyrsten Sinema was projected on Monday night to beat Republican Martha McSally to become the first female U.S. senator in Arizona's 106-year-old history, local media reported.

Sinema will also be the first Democrat Arizona elects to the Senate since 1988.

Sinema's expected victory flips the open seat of retiring Republican Senator Jeff Flake, an open critic of President Donald Trump who won Arizona by 4 points in 2016 presidential elections.

"As long as I've served Arizona, I've worked to help others see our common humanity & find common ground. That's the same approach I'll take to representing our great state in the Senate, where I'll be an independent voice for all Arizonans." Sinema tweeted after the race was called in her favor.

"I wish her success," McSally tweeted. "I'm grateful to all those who supported me in this journey." McSally is a Republican congresswoman and former Air Force pilot.

Sinema has taken a solidly centrist position. She maintained her lead over McSally on Monday, and grew it to 1.7 percentage points, with the latest drop in early ballots, according to a USA Today report.

The Arizona Republic estimates there are about 170,000 votes still to be counted. McSally would have to win those by nearly 23 percentage points to retake the lead. However, 95 percent of the uncounted votes are in two counties where she has underperformed.

Sinema's would-be victory guarantees the Democrats wins at least 47 Senate seats. Republicans control 51, with two still undecided: Florida, where there is a recount, and Mississippi, where there is a runoff, said a New York Times report.