“My vote was motivated by two things: deep concern about Ken Paxton’s record and genuine admiration for the character and principles I see in James Talarico,” Neil Bush wrote on Facebook.
A businessman who has never held political office, Neil Bush is remembered for his role as director of the Silverado Savings and Loan when it collapsed in 1988, costing taxpayers more than $1bn.
Meanwhile, Barbara Bush has donated to help Talarico, campaign finance records show.
Bush, the twin of US chat show host Jenna Hager Bush, reportedly gave $2,500 (£1,900) to a political action committee that supports the Democrat.
A survivor of several political scandals, Paxton was previously impeached by fellow Republicans in the Texas state legislature.
He also beat a primary challenge from the son of Jeb Bush, George P Bush.
The Bushes have had a complex relationship with Trump since he beat Jeb Bush, the former Florida governor, during the 2016 Republican presidential primary race.
Former President George HW Bush previously called Trump a “blow hard” and confirmed he had voted for Democrat Hillary Clinton in the 2016 race.
His son, George W Bush, whom Trump repeatedly lambasted for launching the US war in Iraq, has made thinly veiled criticisms of Trump.
But during his own political campaigns, George P Bush backed Trump.
