The Bush dynasty is no stranger to generational conflict: father and son differed over deposing Saddam Hussein, raising taxes and the role of the United Nations.

Now it is father and daughter who find themselves at odds over a weighty issue.

Barbara Bush, one of the twin daughters of George W. Bush, will endorse same-sex marriage on Tuesday, publicly breaking ranks with a father who, as president, pushed for a constitutional amendment banning such unions.

Ms. Bush, 29, has taped a video calling on New York to legalize gay marriage. A bill to do that was defeated in the state in 2009. She describes the issue as a matter of conscience and equality.

“I am Barbara Bush, and I am a New Yorker for marriage equality,” she says in the brief message, sponsored by an advocacy group. “New York is about fairness and equality. And everyone should have the right to marry the person that they love.”