The first presidential debate between Hillary Clinton and Donald J. Trump this month will exclude two leading third-party nominees — Gary Johnson of the Libertarian Party and the Green Party’s Jill Stein — the Commission on Presidential Debates announced on Friday.

The failure to be invited to the Sept. 26 debate at Hofstra University deals a significant blow to Mr. Johnson and Ms. Stein, who are desperate for national exposure to promote their long-shot bids for the White House. Their running mates will also not be allowed to participate in the vice-presidential debate on Oct. 4, the commission said.

Mr. Johnson and Ms. Stein were excluded because they did not reach an average of 15 percent support in the five national polls that the commission used as a benchmark. Mr. Johnson, a former Republican governor of New Mexico who is polling in double-digits in many state polls, reached 8.4 percent and Ms. Stein reached 3.2 percent.

Candidates from smaller parties have complained to the commission, arguing that the threshold is too high for candidates who lack vast resources or name recognition. Mr. Johnson, who has acknowledged that he has little chance of becoming president if he is not allowed to debate, has started a petition that collected more than 800,000 signatures urging the commission to include him.