Gary Johnson’s online petition to be included in this fall’s presidential debates has almost reached 600,000 signatures—but the powers that be seem unswayed. The Commission on Presidential Debates, which sponsors the televised White House job interviews, is plowing forward with a policy of allowing on stage only candidates who average at least 15% in five national polls.

Using today’s numbers, Mr. Johnson, the Libertarian Party’s nominee, doesn’t make the cut. He has averaged only 8.8%. Neither does the Green Party’s Jill Stein,...