Democrat Kyrsten Sinema's unbreakable hold on Maricopa County, along with her overwhelming support in Martha McSally's Tucson-based congressional district, gave Sinema an edge Republicans could not erase in the race for Arizona's open U.S. Senate seat.

The Associated Press called the Senate race for Sinema on Monday evening and Republican McSally tweeted her congratulations to Sinema.

"I am so honored that Arizonans chose our vision of a better Arizona, and now it's time to get to work," Sinema told supporters gathered in Paradise Valley. "Arizonans had a choice between two very different ways forward. One focused on fear and party politics, and one focused on Arizona and the issues that mattered to Arizona families."

Sinema will be the first woman in the state's 106-year-old history to take a seat on the floor of the U.S. Senate.

Sinema's systematic play for moderate Republican voters, independent voters and suburban women anxious about the polarizing politics in the era of President Donald Trump gave her an advantage in the state's urban areas that was too great for McSally to overcome.

Sinema maintained her lead over McSally on Monday and grew it to 1.7 percentage points with the latest results from early ballots. The results defied the narrative from McSally's campaign that she would perform well enough in the waning days of ballot-counting to remain competitive.

Sinema vanquished Republicans' hopes of hanging onto the Senate seat, which Jeff Flake is vacating.

"I wish her success," McSally wrote on Twitter. "I'm grateful to all those who supported me in this journey. I'm inspired by Arizonans' spirit and our state's best days are ahead of us."

Flake also tweeted his congratulations. "Congratulations to @kyrstensinema on a race well run, and won. It’s been a wonderful honor representing Arizona in the Senate. You’ll be great."

Sinema led McSally by 38,197 votes as of 5 p.m. Monday.

"The Sinema campaign did the math that they needed to do to win and did everything to get there, and didn't allow themselves to get distracted" by responding to Trump's latest tweets or news of the day, Democratic strategist Andy Barr said. "She had a very narrow path and needed to do everything right to win, and did it."

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.

MORE: How the McSally-Sinema race is playing out in Maricopa County

Ninety-five percent of the uncounted votes are in Maricopa and Pima counties, the two places where McSally has underperformed.

Sinema played for crossover voters — those Republicans who may be willing to change their behavior and go for a Democrat — as well as independent voters.

In doing so, Sinema forged a path that allowed her to overtake Pima County, the Democratic-leaning county that anchors McSally's Tucson-based 2nd Congressional District.

Sinema won Pima County over McSally by about 15 percentage points.

And Sinema ran up the numbers in Maricopa County, the state's most populous area, which leans Republican. She won Maricopa County by 3.7 percentage points.

As the latest round of ballots came in Monday, Sinema again tallied big margins in Maricopa and Pima counties, the state's population centers, while McSally piled up small wins in smaller counties.

In Maricopa and Pima counties, Sinema led McSally by 103,000 votes. Elsewhere, McSally led by 64,000 votes.

The Pima County numbers may be especially painful for McSally because most of the county is in her congressional district.

McSally's deficit in Pima County, 54,000 votes, was larger than it was in Maricopa County, which has three times more voters.

Yavapai and Pinal counties are the most populous GOP-leaning areas of the state. Together, they supported McSally by a net 28,000 votes. Mohave County, where McSally collected her biggest share of votes, about 70 percent, gave her another 31,000 net votes.

The state's 10 remaining counties narrowly voted for Sinema.

McSally's biography as the first woman combat pilot and alignment with Trump played well with rural Arizona voters on the outskirts of the suburbs.

But it almost certainly contributed to her cratering in Maricopa and Pima counties, particularly among moderate women and independent voters.