In February, Washington State lawmakers legalized same-sex marriage, a move pushed by Gov. Christine Gregoire, a Democrat. But opponents collected enough signatures to add a ballot measure that would repeal the law. Although most Americans say they support marriage equality, voters in 32 states have rejected such measures at the ballot box.

Bezos, who founded Seattle-based Amazon.com in 1994, is among top Washington State business leaders who have donated in support of the same-sex referendum, including Bill Gates and Steven A. Ballmer of Microsoft, who have each given $100,000.