Story highlights Gary Johnson said he gave almost nothing to charity last year

And he's doubtful about winning Mitt Romney's endorsement for his Libertarian bid

Las Vegas (CNN) Libertarian presidential candidate Gary Johnson said Friday that his charitable contributions are "negligible," bordering on "almost nonexistent."

The former New Mexico governor made the comments to CNN in Las Vegas after addressing a gathering of the Asian American Journalists Association. Earlier in the day, Hillary Clinton's campaign announced that Clinton and her husband, Bill Clinton, donated close to 10% of their adjusted gross income to charity in 2015. And according to CNN's calculation, the couple paid an effective tax rate of 30.6%.

"My charitable contributions would be negligible -- I mean just, really almost nonexistent," Johnson told CNN, as he stressed that his wealth "does not even approach Hillary Clinton's."

Pressed on what percentage of his income he has given to charity, Johnson responded: "For me, giving to charity, I've always viewed that the things that I do in my life contribute to making lives better. That what I've done."

He also estimated that his effective tax rate was around 20%.

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