Rep. Kyrsten Sinema (D) expanded 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 Arizona’s too-close-to-call Senate race, after more ballots from the state’s largest county were released Sunday night.

Sinema has grown her slim lead to 32,640 votes, which is a margin of 1.51 percentage points.

The latest release includes about 36,000 ballots from Maricopa County, the largest county in the state that includes Phoenix, according to Garrett Archer, an analyst for the Arizona secretary of state. There are still about 162,000 votes left to count in Maricopa.

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Overall, there are still 211,000 total outstanding ballots in Arizona. While the majority of those will still come from Maricopa, there’s a significant number of uncounted ballots in Pima County, a Democratic-leaning county that includes Tucson, and Pinal County, where McSally easily won on Election Day.

Political observers told The Hill that they expected Sinema to extend her lead over the weekend, with early votes being tabulated that were likely skewed toward Democrats. They believe McSally is likely to benefit when more votes are released early this week.

The new tallies coming this week, which will include ballots dropped off on Election Day, are likely to favor Republicans, according to observers.

Both campaigns are voicing confidence that they’ll be well-positioned with the votes that are left to be counted. McSally initially had a narrow lead after Tuesday’s election, but Sinema took the lead on Thursday night and has since grown her lead in subsequent ballot releases.

Following the latest vote release Sunday night, Sinema campaign manager Andrew Piatt released a statement calling the Arizona Democrats’ lead “insurmountable.”

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“With the latest ballot count, Kyrsten’s lead is insurmountable. McSally’s campaign said today’s results would be her ‘firewall’ but as we expected, no firewall emerged,” Piatt said in a statement. “This is not plausible. Kyrsten will be declared the next U.S. Senator from Arizona.”

But McSally’s campaign is still confident about their prospects and the additional votes left to be counted.

On Saturday night, McSally campaign CEO Jim Bognet sent a statement that the GOP congresswoman’s vote total “grew at a greater rate than expected,” particularly in rural areas that have mail-in ballots dropped off on Tuesday.

“The latest release provides compelling evidence that the remaining uncounted ballots are favorable to Martha,” Bognet said. “And we will continue our effort to make sure all lawful ballots are counted."

If Sinema wins the seat held by retiring GOP Sen. Jeff Flake Jeffrey (Jeff) Lane FlakeHow fast population growth made Arizona a swing state Jeff Flake: Republicans 'should hold the same position' on SCOTUS vacancy as 2016 Republican former Michigan governor says he's voting for Biden MORE, she’ll be the first Democrat elected to a Senate seat in Arizona since 1988.

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Arizona was one of the few states this cycle where Democrats had an opportunity to flip a seat in their uphill fight for the Senate majority. While it’s traditionally a red state, Arizona went for President Trump Donald John TrumpBiden on Trump's refusal to commit to peaceful transfer of power: 'What country are we in?' Romney: 'Unthinkable and unacceptable' to not commit to peaceful transition of power Two Louisville police officers shot amid Breonna Taylor grand jury protests MORE by less than 5 points in 2016, which is a closer margin than previous GOP presidential nominees.

Regardless of the outcome in Arizona’s election, Republicans have already secured their Senate majority, flipping seats in Indiana, Missouri and North Dakota. But Arizona’s Senate race and the uncalled Florida Senate race — now in the midst of a recount — will decide the size of Republicans’ majority.

Reid Wilson contributed.