Pitt has previously said he won't wed fiancee Angelina Jolie until same-sex marriage is legalised.

In an email to HRC supporters, Brad wrote: "It's unbelievable to me that people's lives and relationships are literally being voted on in a matter of days.

"But that's the system we have and I'm not going to back down from the fight for loving and committed couples to have the ability to marry.

"If you're like me, you don't want to have to ask yourself on the day after the election, what else could I have done?"

HRC president Chad Griffin said the organisation is "proud" to be supported by the Hollywood superstar.

He said: "[Brad's money] delivers vital resources into these campaigns.

"We're proud to be working with him as we show that fundamental fairness will win at the ballot box."

Brad - who raises six children with Angelina - recently admitted he can't see himself marrying until gay marriage is legalised.

Asked about his wedding plans, he said: "I'm still hoping we'll figure out our marriage equality in the States before then."