When Tod Bowman, an Iowa school teacher, asked Hillary Clinton about the future of the US Social Security system at a campaign event earlier this month, he could not have imagined that his question would still be reverberating three weeks later. 

Mrs Clinton refused to say whe­ther she would consider in­creasing taxes to keep the pension scheme solvent – an eva­sion that drew sharp criticism from Barack Obama, her rival for the Democratic presidential nomination.