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Above, we see Texas' elasticity with only swing voters. The state is "difficult, but winnable" -- there are enough metro areas that are elastic to see Biden flipping enough suburban voters to take the state, but it's a tough task.
However, when factoring in turnout variation, we see that, along with general increases in elasticity across the board, the population boom in the Austin area has led to droves of new voters, which is excellent news for Democrats and expands their victory horizon. In particular, Travis (E 68.7), Williamson (E 69.7), and Hays (E 78.0) are key areas that Democrats will look to grow their vote totals in.
Another key set of counties to keep an eye on are the urban areas of Collin (E 65.1), Tarrant (E 41.0), Dallas (E 44.9), and Denton (E 62.1) -- Biden will need big wins here to carry the state in 2020, but the rapid growth in Collin and Denton should serve him well. Evidence of this growth can already be seen in comparing the 2016 Presidential election to the 2018 Senate race; while Clinton lost Collin by 17 and Denton by 20, O'Rourke only lost these counties by 6 and 8 points, respectively, in 2018.
Don't get it twisted -- swing voters are still a huge key to winning Texas. The Rio Grande Valley, in particular, has many conservative Democrats and is extremely elastic, and Biden needs to address his purported weaknesses with Hispanic voters and hold down that area to ensure it doesn't offset his continuing growth in the suburbs.
But when turnout enters the equation (and, as we are seeing, it is rapidly increasing in Texas in 2020), the state becomes way more gettable and goes from "tough, but winnable" to "tossup". It's well and truly in play, because the Democratic areas are gaining a lot of new voters. The surge in turnout we're seeing reflects the elasticity of the areas; Collin and Denton, in particular, have already cast over 35% of their votes with 12 days to go until the election. Stunningly, Harris county (E 43.5) has cast over 50% of its 2016 vote with two weeks to go until the election, and the gains in votes are reflected among young voters and women, two groups that break strongly Democratic. If these are indicators of the results we will get on November 3, it's hard to imagine the race as anything but a tossup, and I would go so far as to call it one in which Democrats are the ever-so-slight favorites.
Rating: Tossup/Tilt Democratic.