Will the Obamas win the election for Harris? Our experts have their say

That argument has not proven effective in winning back support from voters who have switched to Trump over the past four years, many of whom find his own message of national renewal more appealing.

There are also differences in the way Ms. Harris, who has Indian and Jamaican-American roots, thinks and speaks about race.

Mr. Obama has a complex legacy but is rightly remembered primarily as the first black man to reach the White House.

Ms. Harris, meanwhile, avoids the topic of her race for fear of discouraging intolerant voters. The fact that she is a woman – an attribute that Hillary Clinton is making central to her campaign – is also ignored.

It’s also not clear that the Obamas can help Ms. Harris with the biggest problem facing her campaign, namely that voters don’t trust her on the economy or immigration.

Mr. Obama is remembered for passing the Affordable Care Act and other work on Social Security, but his record on immigration is largely forgotten. When the topic comes up in political debate, it is usually to point out that he too was confronted with a high number of illegal border crossings at the US southern border.

His economic credentials, gained through America’s recovery after the 2008 economic crash, do not easily translate to a time when inflation, not growth, is the main topic of debate.

In short, the Obamas are good at reminding their voters that they once supported a progressive, optimistic party that he led and that is now represented by Ms. Harris.

They are unlikely to do her much damage in the election campaign, but her political appeal no longer has the weight it once did. Whatever is wrong with the Harris campaign, the Obamas are not the answer.