Body language expert Patti Wood is weighing in on President Biden and Donald Trump's debate, and says just a few key gestures could impact who wins the war of words.

Patti tells TMZ ... viewers need to look out for 4 first impression factors, including credibility, likability, attractiveness, and power. Patti noted she'll keep a close eye on Biden Thursday night, as she says he's a master at likability -- citing his smile and crinkle around his eyes as evidence.

But, Patti's quick to point out Trump has one very important strength ... which is projecting power.

She says Trump really shines when he channels his anger, with audiences thinking his louder voice and scowl are a show of strength.

According to Patti, the candidates might try to branch out of their wheelhouses a bit. Watch what she predicts Biden might do to take up more space, and why.

To borrow a rather misogynist phrase ... Patti says Trump might wanna try smiling more, and in a particular way. She explains the wry side-smile he's known to flash, won't cut it if he's going for likability.

Patti predicts power will end up being the most important factor ... as both candidates have had their age called into question at different times. Translation, no one wants the Commander in Chief to come off as weak!!!

Patti authored the self-help book, "Snap: Making the Most of First Impressions, Body Language, and Charisma" and tells us these factors come through much more during a debate, than standard political speeches.

Watch the clip ... she says there's a very good reason for that.