Prominent leaders from the Christian right have warned Republicans they must do more to advance conservative values ahead of the US mid-term elections.

Their message to Congress, controlled by Republicans, is "must do better".

Support from about a quarter of Americans who describe themselves as evangelicals was a factor in President George W Bush's two election victories.

The Republicans will need to keep them onboard if they are to retain control of Congress in November.

At a news conference in Washington, some of America's most influential conservative leaders said the current perception among evangelical Christians was that the Republican majority was not doing enough for them.

Tony Perkins, president of the Family Research Council, said that apart from confirming two conservative judges to the Supreme Court, "core values voters" did not feel that Congress was advancing their interests.

The leaders appear to be reflecting a growing sense of frustration among the Christian right, over what they see as a lack of legislative progress on issues such as banning same-sex marriages.

And while this was not quite a call to arms, it will cause concern in Republican circles in the run-up to the mid-terms.

Exit polls suggested that more than three-quarters of white evangelical Christians voted for President Bush in 2004.

But according to a recent opinion poll, the number of them who want Republicans to retain their Congressional majority is not much above 50%.