Play video content EXCLUSIVE TMZ.com

Libertarian presidential candidate Gary Johnson could finally be a viable alternative to Donald Trump and Hillary Clinton ... if he could just get a few key celebs to endorse him.

We got the former New Mexico governer and Libertarian prez hopeful in D.C. and he dished on his campaign strategy.

Check it out if you -- like millions of voters -- desperately want a third option at the polls. Or if you happen to be Taylor Swift or Snoop Dogg -- who might be really interested in Johnson's stand on marijuana.