Mr Dantas is alleged to have been one of the funders of a cash-for-votes scheme in Brazil's Congress mounted by leaders of Lula's Workers' Party in 2003-04.
Mr Karoubi has been ridiculed for proposing to spread oil revenues through cash handouts to the public, but he won only slightly fewer votes than Mr Ahmadinejad in 2005, and claims, credibly, that suspicious polling irregularities accounted for much of the difference.