Throughout Joe Biden’s decades-long career in politics, he has cultivated a reputation of being gaffe-prone, often stumbling on his words – if not outright saying the wrong thing at the wrong time.

That trait is posing fresh challenges for the 2020 Democratic presidential frontrunner amid a series of recent blunders on the campaign trail.

Some Democrats are now questioning if Biden, whose campaign is centered on the notion of electability, is equipped to take on Donald Trump.

“I don’t think people have discerned whether these are gaffes in the classic sense or whether he may have lost his fastball,” said Brian Fallon, a former aide to Hillary Clinton’s 2016 presidential campaign.

“Is this just tripping over your words, as one is sometimes prone to do, or is this a sign of an unsteadiness and an inability to be a commanding presence on the debate stage against Trump?”

There is little evidence thus far that Biden’s “gaffe-machine” persona has evolved into a true liability. He continues to hold a commanding lead in most polls of the Democratic field, with other major contenders trailing by double digits, and his campaign sailing through a series of difficulties largely unharmed.

But Biden, 76, nonetheless invited a new round of scrutiny over his agility following a number of misstatements in the past week alone.

During a swing through Iowa, Biden asserted that “poor kids are just as bright and just as talented as white kids” before quickly correcting himself by adding “wealthy kids, black kids, Asian kids”.

He invoked Margaret Thatcher in comments intended to be about former British prime minister Theresa May, and stumbled over a key stump speech line, telling Iowans: “We choose truth over facts” (as opposed to “truth over lies”).

In the wake of a spate of mass shootings, Biden also recalled meeting with the students from Parkland as vice-president – despite the fact that the 2018 massacre at a Florida high school occurred more than a year after he left the White House.

Biden’s campaign has dismissed the criticism as a fixation of the press, arguing that the gaffes endear him to American voters.

“Joe Biden isn’t overly packaged or poll-tested, and that’s why voters love him,” Biden spokesman TJ Ducklo said in a statement. “He has always spoken his mind and that isn’t changing because he’s running for president or because of how the press chooses to cover him.”

On Tuesday, Biden’s deputy campaign manager, Kate Bedingfield, shared a new poll showing Biden well ahead of his opponents both nationally and among voters in early primary states while accusing the press of “hyperventilating”.

Trump and Republicans have nonetheless seized on the missteps in a bid to raise questions about Biden’s fitness for the presidency.

“Look, Joe is not playing with a full deck,” Trump told reporters at the White House last week. The following day, the president tweeted: “We are ‘playing’ in a very big and complicated world. Joe doesn’t have a clue!”

The attacks echo Trump’s strategy against Hillary Clinton in 2016, which were rooted in rightwing conspiracy theories about her mental fitness.

The attacks on Biden echo Trump’s strategy against Hillary Clinton in 2016. Photograph: Brian Cahn/ZUMA Wire/REX/Shutterstock

Some argue Trump is playing with fire by raising the subject of mental fitness, given the constant scrutiny over his own state of mind and warping of the truth. According to a Washington Post tracker, Trump has made 12,000 false or misleading claims in office.

“Is this really the debate that Trump wants to have? Does he really want to have an argument about who is ‘mentally fit to be president’ and who has a ‘clue’ about what’s happening in the world?” wrote Max Boot, a prominent ‘Never Trump’ conservative columnist.

“Because his own utterances of the past few days have confirmed what everyone who hasn’t joined his cult already knows: He is both unfit to be president and utterly clueless.”

Indeed, much of Biden’s message to American voters has underscored the urgency of limiting Trump’s presidency to a single term. To most Democratic voters, Biden’s gaffes are in no way comparable to Trump’s history of incendiary comments about immigrants and other minorities.

Fallon said Biden’s gaffes, by contrast, were “built into his stock” and seen by many as part of his charm.

“I think he’s got a bit of leeway with Democratic voters on this,” he said, “because the stakes are so high with Trump that I don’t think they’re going to let things that they may think are sort of trivial or media obsessions have undue effect over their preferences.”

However, at the same time, Biden’s Democratic primary opponents have sought to cast him as a relic of the past and out-of-step with an increasingly progressive and diverse Democratic electorate.

The former vice-president weathered an onslaught of attacks in the first two Democratic presidential debates and at times visibly struggled.

Jen Psaki, who served in the Obama administration as the White House communications director and state department spokeswoman, said it was too early to tell if Biden’s gaffes would catch up with his candidacy.

“He says things that are slightly off sometimes. He’s like your ‘uncle Joe’,” she said. “It’s part of who he is and it’s why people think he’s accessible or approachable. I think it becomes more challenging when it feeds into a perception that he may not have the energy or vigor to defeat Donald Trump.”