Gary Johnson, the Libertarian candidate for president, may be on track to win more votes than any third-party candidate in 20 years, if current polling holds up.

Who are his supporters?

Mr. Johnson, a former Republican governor of New Mexico, is relying heavily on the backing of young people and independent voters disillusioned with the two major parties’ nominees.

More than 70 percent of his backers are younger than 50, and over three-fifths are political independents, according to a mid-August poll by the Pew Research Center.

Over all, Mr. Johnson, who will be on the ballot in all 50 states, has the backing of 10 percent of registered voters, the Pew poll found.