In the last American presidential election, in 2012, Mitt Romney was always the favourite to be the Republican candidate. With that accomplished, his sense of self-anointment was such that he appeared to view victory over the incumbent, Barack Obama, as inevitable. He ended up suffering a particularly humiliating defeat. Therein lies the clearest of lessons for Hillary Clinton as she embarks on her campaign for the presidency.

The former secretary of state is all but confirmed as the Democrat nominee for the White House. No credible rival is likely to emerge. She is both experienced and formidable, and will be the toughest of opponents for the candidate selected by the Republicans. But, as with Mr Romney, her apparent strength is also a major potential weakness.

Starting early next year in the Iowa caucuses, the first stage of the primary elections that will decide the Democrat nominee, she needs to convince Americans that she carries no sense of entitlement.

She appears aware of the danger. In a show of humility, her first visit to Iowa is being undertaken in a van. Her pitch is, likewise, to everyday Americans. They, she says, need a champion in the face of growing inequality. At some point, she will also have to differentiate herself from the policies of the Obama Administration to prevent her election being seen as akin to an Obama third term.

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This will be essential because most Americans will be looking for the president they elect in 2016 to be an agent of change. In part, this is a response to the failings of a two-term president who had promised so much.

But it is also related to the reality of American politics. In the past 60 years, the same political party has held the White House for three consecutive terms only once, during the Administrations of Ronald Reagan and George H.W. Bush.

Marco Rubio indicated he understood this when he entered the crowded race for the Republican candidacy last week. The first-term Florida senator said he represented a new generation of leadership, not a familiar face harking back decades like 67-year-old Hillary Clinton. That sentiment has perils also for Jeb Bush, the 62-year-old son and brother of former presidents. But he remains the most likely Republican nominee because he combines experience as a governor of Florida with centrist policies.

If the Republicans are to emerge victorious in what should be an eminently winnable election, they will need a candidate with such credentials. To nominate one whose policies are so far to the right that they satisfy the Tea Party wing of the party is to invite defeat. Mr Romney's fate confirmed as much.

It would be odd for the presidency to again be contested by candidates named Clinton and Bush. Ridding themselves of dynasties was, after all, a central concern of America's founders. Sadly, however, they appear at this stage the best that their respective parties can offer.