Former officials who served in president George W. Bush's administration, including ex-deputy defense secretary Paul Wolfowitz, ex-homeland security secretary Tom Ridge and former secretary of commerce Carlos Gutierrez joined lawmakers and former governors in signing the brief.
The highest court in the United States is set to hear arguments on March 26 against California's ban on same-sex marriage known as Proposition 8, followed by a hearing the next day on the constitutionality of a federal law that defines marriage as a union between a man and a woman.
The brief, organized by the American Foundation for Equal Rights, argues that "the Constitution prohibits denying same-sex couples access to the legal rights and responsibilities that flow from the institution of civil marriage."
The signatories include seven former governors -- Ridge among them -- and members of past presidential campaigns of Bush, Mitt Romney and John McCain.
Eastwood, whose conversation with an empty chair representing President Barack Obama during the Republican national convention sparked controversy, is known as a longtime Republican.
"Many of the signatories to this brief previously did not support civil marriage for same-sex couples; others did not hold a considered position on the issue," the document noted.
But now that several states have approved gay marriage, the signatories, "like many Americans, have reexamined the evidence and their own positions and have concluded that there is no legitimate, fact-based reason for denying same-sex couples the same recognition in law that is available to opposite-sex couples."