PHOENIX — Unpopular among many Americans, Hillary Clinton and Donald Trump have opened the door for a third-party spoiler in the presidential campaign — and just as Gary Johnson is starting to warm up.

Voters like Carlos Moreno could help him catch fire.

“I certainly don’t want Trump to get in, but Clinton worries me,” said Moreno, a registered Democrat who works as a process server in the Phoenix area. “I’ve thought about Johnson, but I haven’t begun reading up on him. I better start.”

The folksy Libertarian, a former Republican governor of New Mexico, is on the ballot in 50 states. Green Party nominee Jill Stein is on the ballot in about half. Neither is remotely within reach of carrying a state. Nor do Johnson or Stein appear to be in a position to tip any states toward Trump.

But there’s a chance that Johnson could move a close race toward Clinton, in much the same way that Ralph Nader pulled enough votes away from Al Gore in 2000 to hand Florida to George W. Bush.

Watch Gary Johnson’s interview on the PBS NewsHour.

Of the roughly dozen battleground states on the road to the 270 electoral votes needed to win the White House, it’s in Arizona where circumstances could align in just such a way that Johnson could play spoiler.

While Trump naturally has an edge as the Republican nominee — the GOP has carried the state in 11 of the past 12 elections — Johnson could steal enough votes away to allow Clinton to snag the state’s 11 Electoral College votes.

In congressional elections in Arizona four years ago, Libertarian candidates drew enough votes away from GOP candidates to allow Democrats Ann Kirkpatrick and Kyrsten Sinema to win a pair of U.S. House races.

The state’s junior senator, Jeff Flake, who had endeared himself to many Libertarians while serving in the House, won his bid for Senate that year, too.

“It could happen,” said Flake. “Donald Trump has managed to make this an interesting state in terms of presidential politics, and not in the way that Republicans have wanted.”

But the politics of third-party spoilers are complicated.

In an August CNN poll conducted in Arizona, likely Republican voters were slightly more likely than Democrats to say they’ll support Johnson if he’s on the ballot, 10 percent to 4 percent.

But Trump’s advantage over Clinton in the poll actually widened slightly when Johnson was included, from 5 points to 7 points. That could indicate that Johnson gives a home to voters who feel closer to the Republican Party, but are not planning on supporting Trump even in a two-person race.