A Fox News national poll showing Trump down 5 is one of his better results lately; keep in mind that Fox News polls hasn't had particularly good news for Trump so far this cycle. But a couple of things to keep in mind.

https://static.foxnews.com/foxnews.com/content/uploads/2020/09/Fox_September-7-10-2020_Complete_National_Topline_September-13-Release.pdf

First, we've had very abundant polling over the past two weeks, both on the national level and also in many states. That makes any one poll less influential. If the race is tightening—or Biden's lead is expanding—the model will need to see that consistently over several polls.

Second, it's mid-September, which means the point where polls start to hone in on a more accurate result. So the mere passage of time helps Biden. This is especially so in our model since it expects the race to (slightly) tighten; so any day of non-tightening is good for Biden.

A slight complication is that we're coming out of the period where you could attribute Trump gains to a convention bounce, though we're not completely out of it yet. So if Trump has "permanently" chopped a point or so off Biden's lead (from 8/9 to 7/8) that's something for him.

Even so, Trump needs the race to tighten more quickly than that. A candidate with a 7-8 point deficit post-conventions is a heavier underdog than one with a 8-9 point deficit pre-conventions, because the reduction in uncertainty from the passage of time outweighs the small gain.