They will paint the picture that Trump is wrong, while giving the impression that everything Biden says is factual.
The fact is, that their fact check claim will be a lie.
It's done to sway people who can be swayed, and hope they never learn any differently that Trump was right and that they had lied with their fact check.
That was done against Mitt Romney when he debated Obama, and Romney just allowed them to get away with the lie. He didn't even challenge that the moderator said his answer was wrong. If I remember correctly, the topic was Russia and their threat to the U.S, and the moderator chimed in that Russia was no longer a threat to this nation. The audience had a good laugh. Two years later we saw the Russia certainly had the capability of being a threat, if the U.S. threatens them with continued NATO expansion, especially after they execute a coup d'état on the sitting Ukrainian president, and they retaliated with the retaking of the Crimea.
Mitt Romney did that to himself instead of simply saying I stand by my statement he let Crowley correct him in real time. Mitt was just a bad candidate when he literally made his convention about him being more likeable and then he adds a Chris Christie as keynote speaker whom literally didn’t mention Mitt Romney’s name till 20 minutes into his speech.
Trump is Trump if he thinks Joe is lying he’ll say so and if he thinks the moderators are protecting him he’ll make sure to point that out too.