Last week we explored the shift in straight-party voting advantage in Republican-held districts toward the Democrats. Today we explore each party’s roles in those shifts.

Between 2012 and 2016, Democrats either cut into the Republican straight-party advantage or seized the advantage in 66 Republican-held legislative and congressional districts, 63 of which are on the ballot this year. Rep. Matt Rinaldi’s (R-Irving) HD115 and Rep. Rick Miller’s (R-Sugar Land) HD26 experienced the greatest “blue shifts” when measured as a percent of all votes cast in their district in 2016. In Rinaldi’s district, the straight-party advantage favored Republicans in 2012 by 5,471 votes. Four years later, the Democrats had an 1,835-vote advantage, a shift of 7,306 votes, equivalent to 12.4% of all votes cast in HD115 in 2016. Put another way, the shift in straight-party advantage is equivalent to one of out every eight voters going from casting a straight-party Republican (SPR) vote in 2012 to a straight-party Democratic (SPD) vote in 2016.

So, what actually happened in HD115? In 2016, 5,069 more people voted SPD than in 2012 and 2,237 fewer people voted SPR than four years earlier. Nearly a third of the “blue shift” in HD115 was caused by a decline in SPR voting.

Republican declines contributed to the “blue shifts” of 27 Republican-held districts, ranging from 2% of the shift all the way to 70%.

In 10 districts, the dropoff of SPR voters represented to at least one third of the overall straight-party shift toward the Democrats. The two Bexar Co. House districts, represented currently by Speaker Joe Straus (R-San Antonio) and Lyle Larson (R-San Antonio), are in the top three. Both of those districts are still redder than the state as a whole, although the open HD121 has been slowing drifting blueward. It was 13% redder than the state as a whole in 2002 but just 2.5% redder in 2016. The other district in the top three is HD94, a Tarrant Co. district represented by Rep. Tony Tinderholt (R-Arlington). It too has remained redder than the state as it drifts slowly blueward.

The highest competitive district on this chart is HD114, which became open when Lisa Luby Ryan (R) ousted Rep. Jason Villalba (R-Dallas) in the Republican primary. Declining SPR voting is equally responsible for the shift in straight-party advantage as increasing SPD voting. In 2016, HD114 voters cast 2,539 fewer SPR votes, a drop of 11% from 2012, and 2,560 more SPD votes, an increase of 15%. The raw vote decline is the third largest of any Republican-held state House district, trailing only HD121 (4,573) and HD122 (4,227). The district has been bluer than the state since 2008, and getting bluer, but Villalba outperformed other Republicans in his district by more than 5% in 2016. Ryan likely does not receive such a boost relative to other Republican candidates.

Nine of the top 16 districts in that chart are located wholly or mostly in Dallas Co. Across the county, 13K fewer SPR votes were cast in 2016 than in 2012, while SPD votes increased by 29K. The seven Republican-held state House districts in the county accounted for 82% of the countywide SPR decline and 81% of the countywide SPD increase.

Sen. Don Huffines’s (R-Dallas) district included 70% of the countywide SPR decline, as 9% fewer district residents voted SPR in 2016 than in 2012. The Dallas Co. portion of U.S. Rep. Pete Sessions’s (R-Dallas) district included 55% of the countywide decline. An increase of nearly 1K SPR votes in Collin Co. partially offset the decline of 7,677 SPR voters in Dallas Co.

Declining SPR voters accounted for at least 30% of the straight-party advantage shift in HD114 (50%), HD102 (43%), HD112 (40%) and HD115 (31%). Those districts are represented by Villalba, Rep. Linda Koop (R-Dallas), Rep. Angie Chen Button (R-Richardson) and Rinaldi, respectively. This is significant because it indicates not only growing Democratic strength but also a dissatisfaction with the Republican brand, at least to the point that a chunk of otherwise reliable Republican voters opted not to cast a straight-party vote.

The likeliest explanation is President Trump. As the Republican presidential nominee in 2016, Trump lost HD114 by 9%, lost HD102 by 10%, lost HD112 by 1% and lost HD115 by 8% to Democratic presidential nominee Hillary Clinton. He also lost Republican-held HD105 by 9%, HD108 by 6% and HD113 by 2%. All seven of these districts, which were won by Republicans, experienced a drop in SPR voters from 2012. While Trump is not on the ballot this year, the election, if it becomes a referendum on his presidency, could see declines in SPR voters and increases in SPD voters relative to 2014. The question will be whether this has down-ballot implications for state House races, particularly for races where Republican men face Democratic women.

Prior analyses have looked at districts that are bluer than the state as a whole, districts won by Hillary Clinton in 2016 and districts where challengers have raised at least half as much as the Republican nominee. We also previously published our high-level analysis of federal and state and legislative seats we rate “lean Republican” or “toss up.”