Nevada: 2,940,058
Colorado: 5,540,545
New Mexico: 2,081,015
Iowa: 3,134,693
Wisconsin: 5,778,708
North Carolina: 10,146,788
Virginia: 8,411,808
Florida: 20,612,439
New Hampshire: 1,334,795
Ohio: 11,614,373
Missouri: 6,093,000 (not really a swing state)
Michigan: 9,928,300
Pennsylvania: 12,784,227
Total population of these swing states: 100,400,749
Population of the United States: 325,365,189
Assuming proportions of eligible voters are similar, 30.9% of the population matters.
Thanks for contributing! I will add that only 50.00001% of each of those populations is required, with no additional benefit for overshooting, so in some ways, only 15.46% of the population matters.