The only undecided voters that matter are those in the states that are still tossups (e.g., Virginia, Florida, North Carolina). How many of those voters are there? What percentage of the undecided in each state does each candidate need to win (does one candidate only need 40% of the undecided voters, while the other needs 65%)? And what is each candidate’s chance of achieving that goal?
Election outcomes are not just a matter of people choosing sides and trying to win the shrinking pool of undecided voters, because elections don’t involve mind-reading they involve actually voting. So elections are about both turnout and persuasion.
This year is considered a turnout-focused election because undecided voters are about 5% of the electorate this year, give or take a point, which is smaller than usual. That proportion is about the same in swing states and non-swing states. Since no candidate ever wins all the undecideds, they’re probably only worth at most a net point or two this year.
Whether that point or two matters depends on how close things get, which turns much more on how well-mobilized each side’s base is. The Democrats have a larger base, but it votes less.
In terms of the candidate’s chances, I recommend the following:
Princeton Election Consortium (Dem)
Election Projection (GOP)
538(non-partisan but Dem-leaning)