After he won the Democratic crown in 2008, Barack Obama told friends that if Hillary Clinton succeeded him as president “I will have failed”. Things have come a long way since then. Now Mr Obama and Mrs Clinton will sink or swim together.

If Mrs Clinton takes the White House in 2016, Mr Obama’s legacy will be safe. Equally, if the president’s ratings continue to languish in the next three years, Mrs Clinton’s ambitions could be dashed. Each needs the other. Like it or not, their fates are tied.

Some might see this as a fairy tale union of once deeply embittered rivals. But the downsides are readily apparent. Mr Obama’s 2008 quip stemmed from the fact that he had been elected on the promise that he would break the mould of US politics. Mrs Clinton represented the old Washington rule book – the cynical game-playing that was personified by Bill and Hillary. Mr Obama offered something fresh. His youth-infused campaign played Silicon Valley start-up to Mrs Clinton’s tired old brand.