Don Lemon says the most detrimental presidential candidate Americans could elect isn't Donald Trump ... but rather Nikki Haley.

The former CNN host joined us Friday on "TMZ Live," and said he thinks there's good reason to sound an alarm on Haley's campaign ... saying she's a more dangerous candidate than Trump.

Don says it's a case of the devil you know versus the devil you don't ... calling out Haley for being unclear on her views, flip-flopping on Trump's insurrection and ripping her track record on abortion and racism.

The way Don sees it ... we've already survived one Trump presidency, and Haley presents a new set of unknowns that might end up being worse than anything Trump ever did.

It's another shot at Haley from Don ... as we reported, he blasted her on social media Thursday for leaving slavery out of her answer on the cause of the Civil War.

Haley's apologized and asked for grace, something Don says is hypocritical.

Remember, Don and Nikki had beef when he was at CNN ... he called her past her prime, and says she showed him no grace for his poor choice of words ... which he's said was just a prime case of misspeaking on live television.

Don's speaking out on political issues for the first time since leaving his post at CNN ... and he says he can finally speak his mind about all candidates, including Republicans.

While he says he doesn't want Trump back in office, and Haley might be worse ... he tells us why Chris Christie is earning his respect, even though he disagrees with most of his policies.

Don's outspokenness here sounds like a sign of things to come -- he broke some news with us about what he's planning professionally for the immediate future.