Paul Lewis, left, interviewing Gary Johnson. The Guardian

Gary Johnson, the Libertarian presidential candidate, tried to change the subject to marijuana legalization when a reporter pressed him on his tax policy in an interview, published Thursday, that grew increasingly heated.

"Well, I'm an idiot," Johnson said, sarcastically opening the interview with The Guardian's Paul Lewis. "Really, I'm the dumbest guy you've ever met in your life."

Johnson was alluding to the fact that interviewers have continued to harp on his publicized gaffes in recent weeks, including being unfamiliar with Aleppo, Syria's largest city, and not being able to name a single foreign leader he admired.

After a contentious discussion of Johnson's poll numbers — he's hovering around 5.8% in the RealClearPolitics average — Lewis turned the conversation to the Libertarian nominee's tax policy.

Johnson said that he wanted some "certainty" that taxes wouldn't go up. He launched into a defense of his record of not raising taxes "a penny" as a two-term governor of New Mexico.

"We're not getting elected king or dictator," Johnson said. "If Congress passes tax reduction, tax simplification, I sign on to it. But I also recognize that government picks winners and losers, that crony capitalism is alive and well."

Johnson's campaign, however, has said that his administration would abolish the IRS and replace all income and payroll taxes with a single consumption tax.

When Lewis said "most of the world's major economists" think Johnson's tax plan wouldn't work, a visibly angry Johnson responded that "he didn't want to argue," and tried to steer the conversation toward marijuana legalization.

"I came out for the legalization of marijuana — let me just use that as an example," Johnson said. "And I will tell you that I had people in my face for years, and years, and years, telling me about how stupid and how idiotic it was that we should allow marijuana to be legal."

"What's that got to do with your tax policy?" Lewis asked.

"It's leadership," Johnson said.

Joe Hunter, the communications director for Johnson's campaign, emailed Business Insider a statement:

"Gov. Johnson is not a wallflower. He has done thousands of interviews, and yes, sometimes they get contentious, especially when questions come with unfair presuppositions. The Fair Tax is but one example. The reporter suggests no "leading" economists agree with it. If it's such a bad idea, why does it have dozens of congressional sponsors and why has it been supported by such fiscal conservatives as Mike Huckabee and Herman Cain? When an interview becomes a debate, it should be no surprise that the gloves come off."

Watch the full video from The Guardian: