Rep. Kyrsten Sinema (D) has taken a narrow lead over Rep. Martha McSally Martha Elizabeth McSallyTumultuous court battle upends fight for Senate Grassley, Ernst pledge to 'evaluate' Trump's Supreme Court nominee The Hill's Campaign Report: Biden goes on offense MORE (R) in a hotly contested race for a U.S. Senate seat in Arizona after a wave of ballots from Maricopa County broke heavily toward the Democrat on Thursday night.

Now, with about half a million votes left to count, Republicans are nervous that the votes left to be counted will favor Sinema.

Sinema leads McSally by just 9,163 votes out of nearly two million cast, a margin of just 0.48 percentage points. Angela Green, the Green Party nominee, has 43,958 votes.

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Publicly, McSally’s campaign says it is confident that the remaining ballots will break their way. McSally's team has told allies that the ballots counted on Thursday are likely to have come from the final early votes cast before Election Day, voters that are most likely to favor Democrats.

The remaining votes to count, McSally's campaign believes, will come from Republican voters who turn out regularly, who dropped their ballot off at polling places on Election Day. Those voters, McSally's team hopes, will put them back in front.

“With half a million ballots left to count, we remain confident that as votes continue to come in from counties across the state, Martha McSally will be elected Arizona's next senator,” McSally campaign chief Jim Bognet said in a statement.

But privately, half a dozen Republican strategists, party officials and state legislators on Thursday and Friday all said they worried a Senate seat was about to slip away. They are closely watching the next tranche of results, due to be released by Maricopa County officials Friday afternoon.

“It goes without saying that if Sinema gains significantly again in Maricopa County today, the race is over,” one party strategist who asked for anonymity to offer a candid assessment told The Hill. “Sinema's jump in the numbers last night defied expectations and all logic.”

Several Arizona political experts said they expected Sinema's lead to grow Friday, when the last of the early votes are counted, and for McSally to begin cutting into Sinema's lead on Sunday, when ballots turned in on Election Day are counted.

“It's highly likely you're going to see Sinema make a small gain today. However, with the Election Day voters, you'd expect Martha to probably claw back a little,” said Mike Noble, a Republican pollster in Phoenix. “It will tighten back up again, but is there enough to swing back?”

A McSally spokeswoman did not immediately respond to a request for comment Friday morning.

In their own statement released Friday morning, Sinema's campaign said it was confident she would hold her lead.

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The vast majority of the remaining ballots to be counted are in Maricopa County, the state's largest, where elections officials are working through about 345,000 uncounted votes. The county is closely divided; Sinema holds just a 2.5 percentage point lead over McSally.

But the next-largest chunk of uncounted ballots, 80,000, comes from Pima County, the more liberal home of Tucson, where Sinema leads by 13 percentage points so far.

The largest pool of votes likely to favor McSally will come from Pinal County, where an estimated 30,000 votes remain uncounted. There, McSally leads by 14 percentage points.

There are 10,000 votes left to be counted in Coconino County, a heavily Democratic bastion based around Flagstaff, and a combined 15,000 votes are left to count in three small rural counties where McSally leads, according to Garrett Archer, a data expert who closely studies Arizona politics.

Election administrators in several counties, including Maricopa, are calling voters whose signatures appear not to match those on file, to verify whether those voters actually cast votes. Republican Party organizations from four counties sued Thursday to try to force a uniform set of procedures across the state, one that would either shut down the process in Maricopa County or force other, smaller counties to engage in their own so-called ballot rehabilitation process.

A Maricopa County judge ruled that counties should continue tallying votes, though he set a follow-up hearing on Friday.

If Sinema does pull off the comeback, McSally still has a path to the Senate. Several Republicans said that Gov. Doug Ducey (R) could appoint McSally to the seat held by Sen. Jon Kyl (R), who was appointed himself to replace the late Sen. John McCain John Sidney McCainCindy McCain endorses Biden: He's only candidate 'who stands up for our values' Biden says Cindy McCain will endorse him Biden's six best bets in 2016 Trump states MORE (R) earlier this year. Kyl has made clear he does not want to stay in the Senate beyond the end of this year.

If McSally and Sinema were to serve together, they would not be the only pair of senators who had run against each other before.

Sens. Angus King Angus KingGovernment watchdog recommends creation of White House cyber director position Democrats step up hardball tactics as Supreme Court fight heats up Shakespeare Theatre Company goes virtual for 'Will on the Hill...or Won't They?' MORE (I-Maine) and Susan Collins Susan Margaret CollinsSenate GOP aims to confirm Trump court pick by Oct. 29: report The Hill's Campaign Report: GOP set to ask SCOTUS to limit mail-in voting Senate GOP sees early Supreme Court vote as political booster shot MORE (R-Maine) faced each other in a race for governor (King won). Sen. John Hoeven John Henry HoevenDavis: The Hall of Shame for GOP senators who remain silent on Donald Trump Bottom line Bipartisan senators seek funding for pork producers forced to euthanize livestock MORE (R) beat Sen. Heidi Heitkamp Mary (Heidi) Kathryn HeitkampCentrists, progressives rally around Harris pick for VP 70 former senators propose bipartisan caucus for incumbents Susan Collins set to play pivotal role in impeachment drama MORE (D) when they ran to become governor of North Dakota in 2000. And Sen. Mike Enzi Michael (Mike) Bradley EnziChamber of Commerce endorses McSally for reelection Cynthia Lummis wins GOP Senate primary in Wyoming The Hill's Convention Report: Democrats gear up for Day Two of convention MORE (R) beat out Sen. John Barrasso John Anthony BarrassoGOP senator attacks Biden: 'I'm not sure what he recalls' Oil and gas is a partner — not an adversary — in meeting our economic and environmental goals Murkowski: Supreme Court nominee should not be taken up before election MORE (R) for the Republican nomination for his Senate seat in 1996, a decade before Barrasso himself got to the Senate.

Republicans have already secured the Senate majority, with wins in Indiana, Missouri and North Dakota. But if they lose Arizona, they could be governing in a small majority again.

Arizona isn't the only uncalled race, with the Florida Senate race between Sen. Bill Nelson Clarence (Bill) William NelsonDemocrats sound alarm on possible election chaos Trump, facing trouble in Florida, goes all in NASA names DC headquarters after agency's first Black female engineer Mary W. Jackson MORE (D) and Gov. Rick Scott (R) likely headed to a recount. And Republicans are looking to defend Mississippi, where they are highly favored to win a special election runoff on Nov. 27.

Lisa Hagen contributed.