Hillary Clinton stepped up pressure on Wednesday to hold fresh primary elections in Florida and Michigan as Barack Obama extended his lead in the popular vote and among elected delegates following his thumping victory in Mississippi on Tuesday night.

Mrs Clinton was also dealt a fresh blow when Geraldine Ferraro, the former vice-presidential candidate, resigned from the finance committee of her campaign for the Democratic party’s presidential nomination after having said that Mr Obama had got to where he was only because of his colour.