Two top Republicans in the Michigan governor’s race face disqualification over forged petitions, officials say.

Both Mr. Craig and Mr. Johnson were widely viewed as front-runners for the party’s nomination in a key battleground state, where Republicans have clashed with Democrats over the legitimacy of the 2020 presidential election and pandemic restrictions.

More than half of the 21,305 signatures submitted by Mr. Craig’s campaign were rejected, leaving him with 10,192 valid signatures, the bureau said in its report, which noted that little effort was made to vary handwriting.

“In some cases, rather than attempting varying signatures, the circulator would intentionally scrawl illegibly. In other instances, they circulated petition sheets among themselves, each filling out a line,” the bureau said of the petitions for Mr. Craig.

Mr. Craig identified Vanguard Field Strategies, an Austin, Texas, firm, as helping to manage the canvassing effort, one that he said relied on several subcontractors that were previously unknown to him. He said that the onus was on the firm to have checks and balances to detect fraud, and he called it “shortsighted” and unrealistic to expect that a busy candidate would verify more than 20,000 signatures.

Vanguard Field Strategies confirmed on Tuesday that 18 of the people identified in the elections bureau’s report as circulating the fraudulent petitions had been working for another firm that it had subcontracted to help it gather signatures. The company would not identify the subcontractor, which it characterized in a statement on Tuesday as a nationally respected Republican firm.

“The allegations of fraudulent activity, and individuals infiltrating Chief Craig’s campaign in an effort to sabotage it, is very concerning,” Joe J. Williams, Vanguard’s president, said in a statement. “I hope the individuals charged with fraud (none of which worked for or were paid by Vanguard) are held responsible if the allegations are true.”