By CBS3 Staff

PHILADELPHIA (CBS) – There is an important deadline Tuesday in the still too-close-to-call Republican U.S. Senate primary between Mehmet Oz and Dave McCormick. Unofficial returns are due from Pennsylvania counties at 5 p.m.

If counties don’t have final results, they must tell the secretary of state’s office how many votes are still outstanding.

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Meantime, the McCormick campaign filed a lawsuit Monday over some of the mail-in ballots from the election. The lawsuit asks the state’s commonwealth court to require counties to promptly count mail-in ballots that don’t have the required handwritten date on the return envelope.

The campaign says at least two counties, Allegheny and Blair, suggested they would not count the ballots.

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The secretary of state said Tuesday any ballot without dates that arrived in time must be counted, but they should be kept separate because of anticipated legal appeals.

As of Tuesday afternoon, close to 99% of the votes were counted. Oz leads McCormick by less than 1,000 votes.

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The race appears to be heading to a recount.