Presidential candidates' sexual relations -- or lack thereof -- with porn stars are now a topic of national debate ... yes this is Trump vs. Biden 2.0.

Stormy Daniels found herself smack in the middle of Trump and Biden during Thursday's debate ... when Biden started taking shots at Trump's felony conviction.

It all started when Biden told voters, "The only person on the stage that is a convicted felon is the man I’m looking at right now," a clear reference to Trump being found guilty of making hush money payments to Stormy.

In the soundbite of the night, Trump shot back ... "I didn't have sex with a porn star!!!"

Yes, this is where America is in 2024.

Trump's the first former U.S. president convicted of a felony, and he's going up against Biden, who is still going to be dogged by questions about his age after his debate performance.

Before the old folks got all fired up over Stormy, it was mostly a dull debate ... aside from Trump pouncing the moment Biden struggled to hold it together.

When Biden seemed to lose his train of thought and trail off, Trump took his shot ... "I really don't know what he said at the end of that sentence. I don't think he knows what he said either."

Something tells us there's more ammo for Stormy's stand-up act here.