75% of the Texas vote is in the 26 largest counties, so a small change in the rural vote helps in only a small way. Cruz really has no upside among rural voters, so any change will be to Beto's benefit.
Here is a list of those counties showing the number of Republican votes in 2016 and 2012, the Republican percentage of the total vote in 2016 and 2012, and the percentage lost by the Republicans (except for Jefferson Co.) (and gained by the non-republican candidates). I also make some rough predictions for 2018.
GUADALUPE 36,632 33,117 63.4 66.9 -3.5
COMAL 45,136 39,318 72.6 76.3 -3.7
ELLIS 44,941 39,574 70.6 73.1 -2.5
BRAZOS 38,738 37,209 57.6 66.5 -8.9
HAYS 33,826 31,661 46.9 53.7 -6.8
MCLENNAN 48,260 47,903 61.0 64.3 -3.2
SMITH 58,930 57,331 69.5 72.0 -2.5
JEFFERSON 42,862 43,242 48.9 48.8 0.1
CAMERON 29,472 26,099 32.0 33.9 -1.9
BELL 51,998 49,574 54.7 57.5 -2.8
LUBBOCK 65,651 63,469 66.3 69.6 -3.3
NUECES 50,766 48,966 48.6 51.0 -2.3
BRAZORIA 72,791 70,862 60.1 66.4 -6.3
GALVESTON 73,757 69,059 60.0 62.7 -2.7
HIDALGO 48,642 39,865 28.0 28.6 -0.6
WILLIAMSON 104,175 97,006 51.3 59.4 -8.1
MONTGOMERY 150,314 137,969 73.5 79.7 -6.2
EL PASO 55,512 57,150 25.9 33.1 -7.2
FORT BEND 117,291 116,126 44.8 52.9 -8.2
DENTON 170,603 157,579 57.1 64.9 -7.8
COLLIN 201,014 196,888 55.6 65.0 -9.4
TRAVIS 127,209 140,152 27.1 36.2 -9.1
BEXAR 240,333 241,617 40.8 47.0 -6.3
TARRANT 345,921 348,920 51.7 57.1 -5.4
DALLAS 262,945 295,813 34.6 41.7 -7.0
HARRIS 545,955 586,073 41.6 49.3 -7.7
2018 Senate Race
Beto (>60%) - Harris, Dallas, Bexar, Travis, El Paso, Hidalgo, Cameron
Beto (53-60%) - Fort Bend, Nueces, Jefferson, Hays
Beto (50-53% - Tarrant, Williamson (Flipped to Democrats)
Cruz (50-53%) - Collin, Denton, Bell (Will Flip in 2020)
Cruz (53-60%) - Galveston, Brazoria, Mclennan, Brazos, Guadalupe
Cruz (>60%) - Montgomery, Lubbock, Smith, Ellis, Comal