Former US republican presidential hopeful Mike Huckabee has slammed opponents of this weekend's controversial meeting of the World Congress of Families in Melbourne, accusing "sexual radicals" of launching a smear campaign.

Mr Huckabee, a former governor of the US state of Arkansas, is a signatory to an open letter about this Saturday's conference, which features speakers who have linked breast cancer with abortion.

The conference is being attended by senior Liberals, including federal Social Services Minister Kevin Andrews and Victorian Attorney-General Robert Clark.

Last month, The Age revealed that Mr Clark would deliver a "welcome to Victoria" address to the event, whose managing director, Larry Jacobs, has spruiked Russia's "crusade" against homosexuality and its laws banning gay pride demonstrations and "homosexual propaganda".

The meeting, and its attendance by senior Liberals, has been criticised for endorsing homophobia.