Most Republicans in the US would be willing to attend a same-sex marriage of someone they knew, according to a survey.

Former Governor Jeb Bush is among senior Republicans who would not demur if invited to the gay wedding of someone they cared about. Senator Marco Rubio opposes gay marriage but would also attend a gay wedding.

The Reuters/Ipsos poll is a further indication of how rapidly social norms on the issue are shifting as some politicians on the right become increasingly wary of staking out a traditional stance against gay marriage. But too liberal a stance can also be a costly risk, especially when presidential hopefuls must still depend on backing from the right of the party.

According to the poll, nearly six out of ten Republicans would attend a same-sex wedding, compared to eight in ten Democrats and seven in ten independents.

Of American citizens overall, nearly seven in ten would attend a same-sex wedding. Two in ten said they would not.

The was published in a week when the US Supreme Court is examining a legal challenge to laws that prevent same-sex marriage, with justices hearing 18 hours of oral evidence alone so far.

Some Christian leaders are threatenening civil disobedience if Supreme Court decides in favour, according to a document sent to the court, Pledge in Solidarity to Defend Marriage and backed by Franklin Graham.

"We stand united together in defense of marriage. Make no mistake about our resolve," the Christian leaders warned. "While there are many things we can endure, redefining marriage is so fundamental to the natural order and the common good that this is the line we must draw and one we cannot and will not cross."

As well as Graham, those who have signed it include Mike Huckabee, Rick Santorum and Pastor John Hagee.