He is her “secret weapon”, the one who can make her campaign, be the best advocate for her candidacy and offer advice and counsel once she gets to the White House.

Yet as he stumps for Hillary Clinton ahead of key Democratic primaries this month in Nevada and South Carolina, questions are emerging about whether Bill Clinton is still his wife’s best calling card, or whether he is doing more to harm her campaign than help it.

Over the past two weeks, the former president has come under fire for comments criticising both Bernie Sanders, Mrs Clinton’s challenger for the Democratic nomination, and President Barack Obama — remarks some political observers say went too far.

After barely eking out a victory in Iowa and posting a 20-point loss in the New Hampshire primary, Mrs Clinton is under increasing pressure to regain momentum in the national race. While she is expected to do well, especially among black and Hispanic Democrats, there are signs that Mr Sanders is making inroads among some of her key constituencies, particularly young voters and women.

The latest gaffe from her husband came at an event in Memphis this weekend, where Mr Clinton appeared to diminish the historical significance of Mr Obama as the first black president, by suggesting that all Americans were of mixed race. “Unless your ancestors, every one of you, are 100 per cent from sub-Saharan Africa, we are all mixed race people,” Mr Clinton told the audience of Democratic supporters.

He also in the same speech criticised Mr Obama for not being what he called a “change-maker”. “A lot of people say ‘you don’t understand — it [the political system] is rigged now’. Yeah, it’s rigged now because you don’t have a president that’s a change-maker,” he said.

In response, Aaron Camp, a Democratic blogger, wrote: “For [Mr Clinton] to claim that we are all mixed race people . . . is dismissive of the serious problem of racism and racial inequality in this country, and, as a matter of fact, racist.

“If we were all mixed race people, there would be no need for a ‘Black Lives Matter’ movement to address systemic institutional racism and police brutality in this country.” Chris Hayes, the MSNBC news anchor, said Mr Clinton appeared to have gone “off message” with his speech.

The reaction follows a similar response to comments Mr Clinton made about Mr Sanders, whom he has described as dishonest and “hermetically sealed”.

He has also accused supporters of Mr Sanders of sexist and profane language about Mrs Clinton — comments which come amid renewed scrutiny over Mr Clinton’s past behaviour, particularly the allegations of sexual impropriety that have long haunted him.

Donald Trump, the Republican frontrunner, has sought to use the former president’s marital infidelity and alleged sexual misconduct as a way of damaging Mrs Clinton. “Hillary Clinton has announced that she is letting her husband out to campaign but he’s demonstrated a penchant for sexism, so inappropriate!” Mr Trump tweeted late last year.

Writing in Time magazine, Caitlin Flanagan, the feminist writer, said she had supported Mr Clinton in the 1990s but now had her misgivings.

At recent Clinton campaign events, people who shared Ms Flanagan’s views were in the minority. Most attendees said they had come to see Mrs Clinton or to show their support for her presidential bid, rather than to catch a glimpse of her husband.

Gaffes aside, Mr Clinton has sought to keep the spotlight on his wife, a shift from her 2008 presidential campaign when he was accused of overshadowing her.

At campaign stops in Davenport, Waverly and Cedar Rapids, Iowa, voters were not treated to the orator who gave a blockbuster, victory lap speech at the 2012 Democratic National Convention. Instead there was a gaunt 69-year-old whose speeches were softer, more meandering, and lacking some of the verve and rhetorical flourish that Mr Clinton was once famous for.

In Waverly, Mr Clinton told of meeting his future wife for the first time. and relayed her achievements as a tireless advocate for children and for universal health coverage, who had excelled as secretary of state and had the “experience and temperament” for the top job.

“Her immediate instinct is, What can I do to make it better?,” he told the small crowd of supporters.

In Davenport, he gave her a brief introduction before disappearing from the stage and leaving her to occupy the podium on her own. “He [Mr Clinton] wasn’t overpowering — it wasn’t all about him,” said Alison McGaughey, a Davenport community college instructor who was in attendance.

“She is so smart . . . Her knowledge is way up here,” said Lorie Huffman, a retired teacher in Waverly who saw Mr Clinton speak on behalf of his wife.

Asked what she thought of the former president, Ms Huffman wrinkled her nose. “He was a nice president,” she said, “but I am more a fan of Hillary.”