Rep. Kyrsten Sinema (D) extended her lead over Rep. Martha McSally Martha Elizabeth McSallyThe Hill's Campaign Report: Presidential polls tighten weeks out from Election Day Mark Kelly: Arizona Senate race winner should be sworn in 'promptly' New ABC/WaPost poll finds Trump edging Biden in Arizona, Florida MORE (R) in the nail-biter race for a U.S. Senate seat in Arizona, after the state's most populous county released new vote tallies Saturday night.

Sinema expanded her lead over McSally to 28,688 votes out of more than 2.1 million cast, a margin of 1.35 percentage points.

Maricopa County, home of Phoenix, disclosed nearly 68,000 new votes on Saturday. Earlier Saturday, Pima, Mohave and Navajo counties all posted new vote tallies as they work through late absentee ballots and early voting.

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Both Sinema's campaign and McSally's campaign have expressed confidence that the continuing counts will benefit their sides, but recent vote batches have tilted heavily toward the Democrat. McSally led initial counts after Election Day, but Sinema reclaimed the lead Thursday night as new votes were counted.

Saturday's tranche of votes also skewed more heavily toward Sinema. Sinema won Maricopa County by 3 percentage points on Election Day, and she won the Saturday tally by about 7 percentage points.

Four Arizona counties still have a significant number of votes to count, and the vast majority of the uncounted ballots are in Maricopa County. Maricopa officials are likely to have about another 200,000 ballots to plow through before every vote is tallied.

McSally's best chance of making up ground will come from the nearly 27,000 votes left to count in Pinal County, which the Republican won easily on Election Day. But Sinema is likely to lead the remaining ballot counts in Democratic-leaning Pima and Coconino counties, which still have thousands of ballots to count.

Observers told The Hill on Friday that Sinema's lead was likely to grow in vote tallies released over the weekend, when elections officials were tabulating the last votes counted before Election Day. Those votes were likely to skew toward Democrats.

On the other hand, McSally is likely to do better in vote counts released early next week. Those tallies will count ballots dropped off at polling places on Election Day, a group of voters more likely to favor Republicans over Democrats.

A recount is extremely unlikely, even if McSally closes the gap dramatically. Arizona law only allows for a recount if the election results are within 200 votes.

If she holds on, Sinema would be the first Democrat Arizona has sent to the Senate since Dennis DeConcini (D), who last won reelection in 1988. DeConcini retired in 1994, when he was replaced by Sen. Jon Kyl (R), who now holds the state's other Senate seat.