I looked at the counties in Texas with the most votes in 2018 and looked at the Democratic vote advantage and the Democratic voting percentage in the 2018 Senate and 2016 and 2012 Presidential races. The top 30 are below that account for 78.7% of the total vote and 88.5% of the Democratic vote. The top 10 accounted for 58.8% of the total vote and 71.2% of the Democratic vote.
There were 8 million votes in 2012, 9 million in 2016 and 8.3 million in 2018. There is expected to be 10+ million in 2020. The 2 million plus 2020 voters that did not vote in 2018 will come almost entirely from these counties.
In 2018 in the top 10 counties where at least 1.4 million of the new 2020 voters will come from, the percentage was 59.1% Democratic to 40.9% Republican.
County Dem adv. 2018 2016 2012
HARRIS 201,298 57.98% 53.95% 49.39%
DALLAS 240,269 66.14% 60.75% 57.11%
TARRANT 4,308 49.93% 43.14% 41.43%
BEXAR 109,346 59.49% 54.19% 51.56%
TRAVIS 240,494 74.30% 65.77% 60.14%
COLLIN -21,811 46.53% 38.91% 33.49%
DENTON -24,095 45.52% 37.13% 33.35%
FORT BEND 30,976 55.75% 51.39% 46.08%
WILLIAMSON 5,993 50.84% 41.59% 37.90%
EL PASO 100,539 74.40% 69.08% 65.51%
MONTGOMERY -86,127 26.97% 22.40% 19.02%
HIDALGO 57,911 68.81% 68.50% 70.40%
GALVESTON -22,576 39.66% 35.52% 35.89%
BRAZORIA -20,465 40.47% 35.65% 32.25%
NUECES 1,436 50.38% 47.12% 47.63%
LUBBOCK -26,712 35.02% 28.30% 28.81%
BELL -9,020 44.37% 39.79% 41.19%
HAYS 12,276 57.16% 46.04% 43.27%
CAMERON 20,196 62.60% 64.51% 64.99%
SMITH -30,578 29.95% 26.31% 26.95%
MCLENNAN -17,403 38.00% 34.22% 34.47%
JEFFERSON 397 50.01% 48.44% 50.40%
BRAZOS -8,095 43.23% 34.40% 31.23%
COMAL -27,249 27.37% 22.90% 22.22%
ELLIS -21,916 31.53% 25.53% 25.63%
GUADALUPE -13,859 36.79% 31.81% 31.80%
PARKER -34,115 18.27% 14.69% 16.47%
JOHNSON -27,160 23.67% 19.07% 21.49%
WEBB 21,345 71.20% 74.35% 76.56%
RANDALL -28,866 19.84% 15.41% 15.24%