Former Obama speechwriter Jon Favreau is laying out the keys to victory for President Biden, at least when it comes to debating Donald Trump ... and it's more about demeanor than policy.

Jon joined us Wednesday on "TMZ Live" to share some insight on how Biden is most likely preparing for Thursday's presidential debate, and what it's going to take to have a successful night.

With all the concerns, valid or not, about Biden's age, Jon says POTUS needs to bring the energy on stage ... and be forceful in everything he says.

Jon says if the prez accomplishes that, it would go a long way toward calming people's concerns about his strength as the leader of the free world.

Jon used to write Obama's speeches and prepared him for his debates, and he says Biden's camp almost certainly has him running mock debates at Camp David this week ... complete with a stand-in "Trump."

Another key ... Jon says the President should be sure to remind voters about Trump's 2020 election lies and ties to the insurrection.