Republican Senate hopeful Martha McSally, who was locked in a too-close-to-call race with Democrat Kyrsten Sinema, called her opponent and conceded Monday after Sinema's lead widened.

The concession marked the end of a heated neck-and-neck battle between McSally and Sinema over who would fill the seat U.S. Sen. Jeff Flake decided not to defend.

McSally wrote a congratulatory message to Sinema on Twitter Monday.

"I wish her success," McSally wrote. "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."

In a video attached to the tweet, McSally said she just called Sinema to congratulate her on becoming Arizona's first female senator "after a hard-fought battle."

Newly counted votes in Maricopa and Pima Counties helped Sinema grow her lead over the weekend. By 5 p.m. Monday, Sinema led McSally by 38,197 votes, or about 1.7 percentage points.

Sinema will be the first woman Arizona has elected to the U.S. Senate. She will also be the first Democrat elected to serve Arizonans in the Senate since 1988, when three-term Democrat Dennis Deconcini was last elected.

The Arizona Republic estimates there are about 170,000 votes still to be counted. But most of the remaining uncounted votes were in Maricopa and Pima counties, where Sinema has outperformed McSally.

The race between the two U.S. representatives saw massive funding efforts on both sides. The campaign grew increasingly negative over the past few months, with McSally dinging Sinema on her past as a liberal activist and Sinema hitting on McSally's health care positions.

On election night, the race was too close to call, though McSally held a small lead as hundreds of thousands of ballots remained uncounted. But as the week wore on, Sinema eclipsed McSally.

On Sunday, Sinema's campaign manager, Andrew Piatt, said her lead was "insurmountable."

The race is far outside the margin that would trigger a recount. An automatic recount is required when the results from the official canvass show that the difference in total votes cast for McSally and Sinema is less than or equal to the lesser amount of one-tenth of 1 percent, or 200 votes.

The secretary of state is scheduled to canvass the results Dec. 3.

Reporter Yvonne Wingett Sanchez contributed to this story.

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