Clay Aiken knows what it's like to go from beloved celeb to despised politician -- by half of the public, at least -- and he's not sure The Rock or Matthew McConaughey are up for it ... so he's sharing his experience with political haters.
As you may know ... the former "American Idol" sensation ran for a North Carolina Congressional seat in 2014, and though he won the Democratic primary, he got beat pretty soundly in the general election by the Republican incumbent.
Clay joined "TMZ Live" Wednesday to share just a slice of what the experience was like, and it doesn't sound pleasant ... and definitely not something a couple of universally cherished actors would want to go through.
Ultimately, he thinks neither Rock nor McConaughey will opt to run, like Caitlyn Jenner already has -- but Clay's adamant she's already experiencing the blowback ... just for different reasons.
As we reported ... Bill Maher blasted the stars running for office or considering it, and though Aiken's not nearly as blistering with his take, the 2 seem to be on the same page -- just don't do it if fame's what matters.
BTW, Clay's still taking on political and social issues ... on his podcast, 'How The Heck Are We Gonna Get Along.'
For the full interview with the former 'Idol' star, check your local listings for Wednesday's "TMZ Live."