(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.

Congrats to @kyrstensinema. I wish her success. 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. pic.twitter.com/tw0uKgi3oO — McSally For Senate (@MarthaMcSally) November 13, 2018

The concession by McSally means the Senate will have 51 Republicans and 47 Democrats in the upcoming Congress. The Senate race in Florida remains undecided and the Mississippi Senate race is going to a runoff later this month.

The win comes despite President Donald Trump and Republicans pushing false conspiracy theories about why ballots in Arizona were taking so long to count.

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