Johnson, a former two-term Republican governor of New Mexico, is expected to emerge this weekend as the Libertarian standard-bearer.
"Is my door going to be knocked down because I'm going to get checked for papers?" Johnson said while praising undocumented immigrants who are leaders in their communities.
"We're going to now come in and knock down doors and they're going to be deported? It'd be like putting them on the moon," he said from New Mexico.
Johnson accused Trump of spreading "misinformation" about the criminal histories of those who came here illegally, and maintained that many undocumented immigrants fill jobs that Americans don't want. Johnson largely saluted those workers -- calling them "the cream of the crop" -- and said he supported making it easier for them to receive work visas and social security cards.
"Insanity comes to mind," he said of Trump's proposal.
Johnson ran in 2012 as well, receiving about 1% of the popular vote. But the former governor said he is already being taken more seriously this cycle than the last one, adding that the Libertarian Party candidate is the only one who will appear on ballots in all 50 states this November.
And in a close election between two unpopular candidates, Hillary Clinton and Trump, Johnson could be more successful than most third-party hopefuls.
"Just appearing in the polls, I think, has a self-fulfilling prophecy of, 'Well, what is this guy really saying?'" Johnson said.
He also defended his likely vice presidential running mate's comments comparing Trump's immigration plan to Nazi Germany
"Absolutely, absolutely," he said.
Former Massachusetts Gov. Bill Weld -- expected to be picked as Johnson's VP -- told The New York Times on Thursday that Trump's plan to remove the estimated 11 million undocumented immigrants from the U.S. reminded him of "Kristallnacht," or the "Night of Broken Glass."
"I can hear the glass crunching on Kristallnacht in the ghettos of Warsaw and Vienna when I hear (Trump's plan), honest," Weld told the Times.
The 1938 pogrom against European Jewry occurred when anti-semitic mobs burned synagogues, destroyed Jewish-owned stores and killed scores of Jews, but not in Warsaw, as stated by Weld.