Senate candidate David McCormick is under fire from his own party after filing a lawsuit to compel the counting of Republican mail-in ballots without a handwritten date on the outside envelope, part of an effort to defeat GOP primary opponent Dr. Mehmet Oz.

The Republican National Committee confirmed Tuesday it was joining the Pennsylvania Republican Party in opposing a McCormick campaign lawsuit asking the state to force all 67 counties' boards of elections to count GOP mail-in ballots received on time but without the dated exterior envelope mandated by statute. The RNC and the Pennsylvania GOP insist their position does not equal an Oz endorsement — except McCormick trails and needs to expand the pool of available uncounted mail-in ballots to win.

"The RNC is intervening in this lawsuit alongside the Pennsylvania GOP because election laws are meant to be followed, and changing the rules when ballots are already being counted harms the integrity of our elections,” the party’s chief counsel, Matt Raymer, said in a statement provided to the Washington Examiner.

“Either of Pennsylvania’s leading Republican Senate candidates would represent the Keystone State better than a Democrat,” Raymer added. “But Pennsylvania law is clear that undated absentee ballots may not be counted.” Added the Pennsylvania GOP on Twitter: “While the [state party] looks forward to supporting the [nominee,] whoever it may be, we absolutely object to the counting of undated mail-in ballots.”

Even the National Republican Senatorial Committee is against McCormick.

The NRSC has issued a statement chastising the lawsuit his campaign filed late Monday in the Commonwealth Court of Pennsylvania, a state appellate court, although the Senate GOP campaign arm has no plans to join the RNC and the Pennsylvania GOP in taking legal action and claims no favoritism toward Oz. “The NRSC has fought to defend common-sense election integrity measures and supports prior decisions to prevent the counting of undated ballots,” spokesman Chris Hartline said.

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Oz on Tuesday afternoon led McCormick by 932 votes out of more than 1.3 million cast in the seven-person Pennsylvania Republican Senate primary field. Most precincts continued reporting ballot results one full week after the primary election, and the campaign of McCormick, a former hedge fund executive, continues to express confidence that it will prevail once all properly and timely cast mail-in ballots are counted. In a statement, McCormick spokeswoman Jess Szymanski said Republican blowback would not cause the campaign to drop its lawsuit or alter its legal strategy.

“Once we have counted all Republican votes received on time, we will unite behind a strong GOP nominee to defeat socialist [Democratic nominee Lt. Gov.] John Fetterman in the fall. All Republicans should be focused on that goal,” she said. Regardless of who finishes on top, the expected narrow margin separating Oz and McCormick will likely trigger an automatic recount, stretching this process well into June.

McCormick campaign lawyers are basing their lawsuit on a fresh decision by the 3rd U.S. Circuit Court of Appeals emanating from a dispute over a local Pennsylvania election from last year. In that case, the court ruled in favor of counting mail-in ballots submitted on time but without a handwritten date on the outside envelope. The McCormick campaign is asking the state to compel counties to include such ballots cast in last Tuesday’s Republican primary.

McCormick, who performed better than Oz with mail-in and absentee voters, is making the case that all Republican voters should be heard. But the first-time candidate is risking a promising future in GOP politics with his lawsuit because grassroots conservatives, not to mention former President Donald Trump and the party’s governing committees, are highly suspicious of mail-in ballots and opposed to court-enforced changes ordered just prior to and immediately following an election.

In comments that appeared intended to provoke political trouble for McCormick, top Democratic election lawyer Marc Elias needled him on Twitter by declaring support for the legal action by McCormick's campaign to force the counting of undated mail-in ballots. “My team was literally working on this same lawsuit for the November election,” Elias tweeted.

The Oz campaign is using this political dynamic as a weapon to paint McCormick as a sore loser who is using Democratic tactics to cheat his way to victory. “We have the strong legal conserve standing here and McCormick is pursuing the leftwing legal strategy,” the Oz campaign said. The Oz campaign was assessing its legal options, mulling whether to join the RNC and the Pennsylvania GOP in opposing McCormick’s lawsuit.

The Oz campaign is expressing confidence that victory is at hand, regardless of any success McCormick might experience with his lawsuit. However, Oz’s legal team believes the suit is flawed, saying the 3rd Circuit’s ruling related only to a local election held in January 2021 to deal with a dispute related specifically to that election and was not written to apply to other political contests.

“We know all legal options available to us, and McCormick, and feel confident they have no path to victory,” the Oz campaign said.

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Meanwhile, the Oz campaign is batting down accusations from the McCormick campaign suggesting it is trying to “disenfranchise” Republican voters in a bid to maintain its razor-thin lead. The McCormick campaign was referring to a filing by the Oz campaign submitted in Philadelphia County that sought to challenge various provisional ballots in a particular precinct.

The Oz campaign said the motion was not a legal filing but rather a document submitted to county elections officials as part of the “routine adjudications process” intended to preserve its right to challenge illegal ballots — for instance, those that might have been double-cast or might contain a flaw that undeniably disqualifies them under the law.