(CNN) -- Democratic Rep. Kyrsten Sinema will win Arizona's US Senate race, defeating Republican Rep. Martha McSally and flipping a seat that had been in GOP hands for 24 years.
McSally conceded the hard-fought race on Monday night -- six days after the election -- as county-by-county tallies of mailed-in ballots continued to increase Sinema's lead.
"I just called Kyrsten Sinema and congratulated her on becoming Arizona's first female senator after a hard-fought battle. I wish her all success as she represents Arizona in the Senate," McSally said in a video posted on Twitter Monday night.
Sinema's victory showed that Democrats are continuing to gain momentum in the rapidly diversifying Sun Belt and firmly places Arizona among the most important presidential and Senate battlegrounds.
The race was certain to deliver Arizona its first female senator. And it came at a time of major change in the state's delegation.
Sen. Jeff Flake, a critic of President Donald Trump's incendiary rhetoric, opted to retire rather than face an all-but-certain loss to a Trump-aligned Republican in a primary -- opening up the seat on this year's ballot.
Then, in August, longtime GOP Sen. John McCain died. Republican Gov. Doug Ducey appointed Republican former Sen. Jon Kyl to the seat, but Kyl has committed to filling it only through this year -- leaving the possibility that Ducey could have to select another new senator in the coming weeks or months. Either way, McCain's seat is on the ballot in 2020 and is sure to be a top Democratic target.
The upheaval comes as Arizona, long a Republican bastion, emerges as a swing state in presidential elections. Hillary Clinton lost to Trump by just 4 percentage points there. The result, and the state's growing Latino population, solidified Arizona as a battleground moving forward.
The fight between McSally and Sinema was bitter, with both drastically shifting their previous positions.
Attempting to prove her conservative bona fides during a primary fight with former state Sen. Kelli Ward and former Maricopa County Sheriff Joe Arpaio, McSally abandoned her previous support for moderate immigration bills.
Sinema's House record ranks her among the most centrist Democrats -- but McSally seized on her rival's past as an anti-war activist, airing a television advertisement featuring Sinema protesting in a pink tutu. McSally told The Arizona Republic that Sinema "has a lot of explaining to do if you look at her Green Party-pink tutu, proud Prada socialist past and her extreme makeover."
Latino Democratic strategists, meanwhile, groused that Sinema had moved too far right on immigration to appeal to a crucial, growing portion of the Democratic electorate.
The focal point of the contest, though, was health care -- particularly pre-existing conditions. Sinema and Democratic groups lambasted McSally's vote for a House GOP bill to repeal Obamacare, which nonpartisan experts said would weaken the Affordable Care Act's protections for those with pre-existing conditions.
McSally responded with advertisements claiming to be "leading the fight" to ensure those people are protected -- even though, under Obamacare, they already are, and the fight in Congress was over Republicans' efforts to change those protections.
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