LANSING, Mich. (AP) — The top U.S. Senate candidates in Michigan cleared the final hurdle for the August primary ballot on Friday, with a state board confirming they submitted the required number of valid signatures.

Michigan's Board of State Canvassers voted to place top Republican candidates Mike Rogers, Justin Amash and Sandy Pensler on the Aug. 6 primary ballot. On the Democratic side, U.S. Rep. Elissa Slotkin was approved.

Democratic groups contested the three Republican candidates' petition signatures while actor Hill Harper — who is running in the Democratic primary — challenged Slotkin's, but the state board found all had qualified.

All are vying for a seat being vacated by retiring Democratic Sen. Debbie Stabenow. A candidate must turn in 15,000 valid signatures to qualify for the primary ballot, and they can turn in a maximum of 30,000 signatures to reach that number.

Qualifying for the primary ballot has become an obstacle for many campaigns in Michigan in recent years. Multiple high-profile gubernatorial campaigns ended two years ago as a result of signature fraud, and earlier this month a U.S. House candidate was disqualified for the same reason.

One Democratic U.S. Senate candidate, Nasser Beydoun, was disqualified Friday after it was found that the campaign address listed on his nominating petition signature sheets was a post office box, not a street address as required. Beydoun, a Dearborn businessman, was seen as a long-shot candidate, but he can still challenge the decision in court.

Invalid signatures can arise from various issues, such as signers not being registered voters in the district or duplicate entries. In the past, some campaigns have been derailed by large numbers of invalid signatures because of signature-gatherers employing illegal methods.