The Yuma, Navajo, Apache and Maricopa County Republican parties filed a lawsuit last night to challenge Arizona's mail-in ballot-counting procedures. The groups "are challenging the way counties verify signatures on mail-in ballots that are dropped off at the polls on Election Day," according to the Arizona Republic.

Election officials have begun to tally more than 600,000 outstanding votes in the race between Republican Representative Martha McSally and Democratic Representative Kyrstin Sinema, who are both vying to replace retiring Senator Jeff Flake.

75 percent of Arizona voters cast ballots by mail. However, those ballots need to go through the signature confirmation process, a laborious task which could take several days. Once signatures are confirmed, the ballots can be opened and tabulated. County recorders who have issues verifying signatures are then allowed to ask voters to verify their identity.

The four county Republican parties allege that county recorders do not follow a uniform standard that would allow voters to adjust issues with their mail-in ballots. Their lawsuit also alleges that two counties allow those fixes after Election Day. The GOP had threatened to sue before the election. Democrats, however, say these claims amount to little more than voter suppression tactics because recorders have followed the same procedures for years without issues.

“A foundational principle of American democracy and our justice system is that all votes are treated equally,” Arizona Republican Party Chairman Jonathan Lines said in a statement to the Arizona Republic.

"This equal protection under the law is enshrined in our Constitution. It is not fair nor just that voters in one county are treated differently under the law from other voters in Arizona. This suit seeks immediate redress of any inequality between ballots cast across Arizona. We stand behind our local county parties demand for equal treatment," he added.

Arizona Democratic Party chair Felecia Rotellini countered with a statement of her own.

“The Republican party is doing everything it can to silence thousands of Arizonans who already cast their ballots," she said. "That's absolutely wrong, and the Arizona Democratic Party is fully prepared to fight to ensure that every last Arizonan has their vote counted."

Andy Gordon, an election-law attorney who represents Democrats, also criticized the lawsuit.

"It's classic Republican 11th-hour stuff," he said. "There are counties all over the state who have been doing this forever … and so now that we’ve got a different regime down here in Maricopa County, and we’ve got a really, really, really tight Senate race, suddenly the Republicans say, 'You shouldn’t’ be doing this, we need to call it off.'"

"You can’t sit on your hands until we see 'OK, now it’s close now we want to do something about it,'" he added.

Speaking of the signature verification process, Chris Herring, the Maricopa County Republicans chair, insists that county recorders were “treating ballots differently.”

We just want all 15 counties treated the same way," he said.

Maricopa County Recorder Adrian Fontes told the Arizona Republic that he and other county recorders had conducted post-election verification calls for a decade.

"There are voters all over Arizona that have been benefiting from (this practice) for over a decade and Maricopa County voters should have that same benefit of due process," Fontes said.

The most up-to-date numbers from The Associated Press show McSally has a slight lead, with 49.4 percent of the vote and 856,848 ballots counted. Sinema is just behind her, having received 48.4 percent of the vote with 839,775 ballots counted.

A hearing is scheduled for tomorrow. The plaintiffs are seeking an accelerated hearing for later today.