If you take the polls at face value right now—with Biden gaining/holding steady in AZ, WI, MI but losing ground in PA & FL, then Arizona is the tipping-point state right now, which could lead to the map shown here. This is a winning map for Biden, but it's precarious.

Biden's winning 270 electoral votes there. But that assumes he wins NE-2 in that map (where he's been well ahead in D partisan polls but which hasn't had any nonpartisan polling) plus hold on in NV and NH, plus hope there are no faithless electors.

Conversely, winning PA but losing WI would be a lot more robust, because that puts Biden at 280 EV with Arizona. He could lose any 2 out of 3 of the NH/NV/ME-2 group and still have enough electoral votes to win.

With PA + AZ, he could also lose Wisconsin *and* Minnesota if he holds onto the others. Those 20 electoral votes in Pennsylvania matter a lot, which is why it's showing up as the tipping-point state almost 1/3 of the time. https://projects.fivethirtyeight.com/2020-election-forecast/