Democratic U.S. presidential candidate Joe Biden, who last year admitted he can be a ''gaffe machine," is taking some heat for new remarks made on the campaign trail in Iowa.

At a town hall in Des Moines that seemingly was about access and opportunity, Biden unintentionally waded into racial politics, an area he's grappled with in the past.

"We have this notion that somehow if you're poor, you cannot do it. Poor kids are just as bright and just as talented as white kids," Biden said. He quickly added: "Wealthy kids. Black kids. Asian kids. No, I really mean it. But think how we think about it.

"They can do anything anybody else can do, given a shot," said the former vice-president.

The event was presented by the Asian and Latino Coalition, a registered political action committee.

A Twitter account associated with Donald Trump's re-election campaign was quick to seize on the comments, uploading the excerpt from Biden's speech to its account.

Trump, to reporters Friday at the White House, said: "Joe is not playing with a full deck. He is not someone you want as your president."

The president ignored a followup question about his own rhetoric, which has included singling out several non-white Democratic members of Congress for criticism, including that some could "go back" to their countries of origin if unhappy about his administration's policies.

However, fellow Democratic presidential candidate Bill de Blasio, who has biracial children, criticized Biden as well.

"To quickly dismiss @JoeBiden's words as a mere 'slip of the tongue' is as concerning as what he said," de Blasio posted on social media. "We need to have a real conversation about the racism and sexism behind 'electability.'"

Focus on Biden's race record

Biden is the front-runner in the Democratic race according to most polls, but it remains to be seen if that standing is a reflection of the depth of his support or his greater name recognition than all other candidates.

Biden is on his third presidential campaign, after efforts that fizzled early in the campaigns of 1988 and 2008.

He issued an apology in 2007 after describing fellow candidate Barack Obama as "the first mainstream African-American who is articulate and bright and clean and a nice-looking guy."

U.S. Senator Kamala Harris turns her attention to Biden during the second set of 2020 Democratic U.S. presidential debates, in Detroit on July 31. (Lucas Jackson/Reuters)

Obama and his team were undeterred by the remarks a year later, selecting Biden as running mate for an ultimately successfully presidential bid, and the two men have spoken with affection for each other since their eight years together in office.

During this year's campaign, African-American candidates Kamala Harris and Cory Booker have seized upon past legislation on Capitol Hill that Biden helped enact, arguing that in some cases it led to deleterious effects for blacks. Harris questioned his stance on desegregation busing of students in the 1970s, while both Harris and Booker have taken him to task for his role in crafting a 1994 federal crime bill, which they claim helped lead to the disproportionate incarceration of African-Americans.

Biden also took flak earlier this summer when he boasted about his ability to work with all political viewpoints by citing his ability to reach compromises with white segregationist lawmakers four decades ago.

'The words of a president matter'

Biden defended himself of the various accusations, saying any objective analysis of the totality of his record since entering Congress in 1973 would illustrate a dedication to advancing civil rights and opportunities for non-whites.

"When I talk about the Obama years, my opponents talk about it like it was ancient history," he said last month, according to the Los Angeles Times. "When others talk about something in the 1970s, they talk about it like it was yesterday. Strange, isn't it?"

Less seriously in terms of gaffes, Biden on the campaign trial has reportedly twice used the name of Margaret Thatcher when referring to recent events in Britain, when the apparent reference was to recent British Prime Minister Theresa May.

Biden is among three Democratic candidates over age 70, along with Bernie Sanders and Elizabeth Warren. Each, if elected president, would top the mark set by Trump as the oldest at inauguration – Trump was 70 years and 220 days old when sworn into office.

Biden has made questions of character — that of Trump and the nation — the centrepiece of his White House bid. He says it was Trump's equivocating response to the 2017 racial clash in Charlottesville, Va., that prompted him to run and he has repeatedly declared the election a battle "for the soul of the nation."

In the wake of two high-profile shootings in the U.S., Biden reasserted himself this week in Iowa with a blistering takedown of Trump's language and the ways in which some of the Republican president's anti-immigrant outbursts could have inspired one of the shootings.

"The words of a president matter," he said in a Wednesday speech. "They can appeal to the better angels of our nature. But they can also unleash the deepest, darkest forces in this nation."