The Democrat’s lead stands at about 33,000 votes, according to the latest vote count.
Sinema campaign manager Andrew Piatt called the lead “insurmountable.” Piatt said McSally would have to win the remainder of uncounted ballots in Maricopa County by a 22 percent margin to make up for lost ground.
“This is not plausible,” he said. “Kyrsten will be declared the next U.S. Senator from Arizona.”
While McSally appeared to be the favorite on Election Day, Sinema outpaced her late Thursday and has remained ahead.
The razor-thin margin in the traditionally red state has provoked Senate Republicans’ campaign arm into levying factually baseless accusations of vote tampering. In recent days the race has devolved into a dispute over the signatures on mail-in ballots.
Local Republican parties sued country recorders last week to halt their practice of reaching out to a voter when the signature on their mail-in ballot does not match the signature on file before tossing it. Counties will have until mid-week to allow people to “cure” issues with signatures, under an new agreement announced Friday by an Arizona judge.
Watch: Fiery Arizona Debate in 4 Minutes
About 220,000 ballots have yet to be tallied, according to a senior analyst with the Arizona secretary of state’s office. A drawn-out vote counting process is typical in the state, and this election could stretch into Thursday, according to The Associated Press.
A Maricopa County official projected the signature issue will affect less than 10,000 of the outstanding ballots.
Still, Sinema’s expanding lead in recent days has incited some national Republicans — including the National Republican Senate Committee and President Donald Trump — into accusations of fraud without providing any evidence.
“Just out — in Arizona, SIGNATURES DON’T MATCH. Electoral corruption — Call for a new Election? We must protect our Democracy!” Trump said during a Twitter barrage Friday.
The NRSC likened signature matching outreach to “cooking the books” for Sinema in an a media release Saturday.
Efforts by national Republicans to delegitimize the results in Arizona have led to heightened fears among voting rights experts — who say it undermines the democratic process — and have drawn rebukes from Republican operatives with a connection to the state.
John Weaver, a former aide to the late Arizona Sen. John McCain, decried the tactic as “lies & nutty theories,” and criticized the NRSC for “undermining democracy.”
I ran @SenJohnMcCain political world for 10 years & can tell you what is going on in AZ is above board & by the rules. The @NRSC & @SenCoryGardner are not only spreading lies & nutty theories, but undermining democracy. @RepMcSally lost this race when she became Trump clone.
— John Weaver (@jwgop) November 11, 2018
Another former McCain aide, Mark Salter, made the same critique.
“The race is almost certainly lost and nothing will change that,” he wrote. “All this does is poison our politics more. Despicable.”