In this highly polarized election cycle there are very few voters who have not decided whether or not they're willing to live through another 4 more years of a Trump administration. National polls are showing Biden leading Trump by about 7 percentage points. But polls won't determine the outcome of the election, votes will. And more specifically, the votes that get counted will determine the outcome of the election.
The Trump campaign is doing everything it can to keep people from voting, because they know that more that vote, the more likely it is that he'll be forced out of the White House on January 20th. That's why they are trying to undermine mail-in-voting and recruiting armies of "poll watchers," including off-duty law enforcement officers, to intimidate people at the polls.
Much of the administration of elections happens at the county-level. So for protecting voting access that's where the action is. While Trump picks up votes in a large number of rural, sparsely populated counties, Democratic candidates have historically picked up votes in just a handful of urban, densely populated counties.
Over the past five presidential election cycles, Democratic candidates have picked up the bulk of their crucial swing state votes in just a handful of counties. If people in those counties get to vote and their votes get counted then Trump loses. It's as simple as that.
Florida: Broward, Miami-Dade, and Palm Beach
Ohio: Cuyahoga (Franklin county is also often important)
Pennsylvania: Philadelphia and Allegheny (Montgomery county is also often very important)
Wisconsin: Milwaukee and Dane
Don't take our word for it, check out this interactive map with election data from the past 5 cycles.