Washington DC - Donald Trump was struggling Monday to contain a fierce backlash to racist rhetoric targeting Puerto Ricans at his weekend rally, just eight days ahead of a presidential election that could be determined by the Latino vote.

The Republican was seeking to spell out his closing pitch and energize his base with the extravaganza at New York's Madison Square Garden but made news instead for a series of crass remarks from allies giving warm-up speeches.

Residents of Puerto Rico, an American island territory in the Caribbean, cannot take part in US elections but the diaspora living in the US numbers almost six million, according to Pew Research Center, and is eligible to vote.

They could have significant sway in the critical battleground state of Pennsylvania, which has the fourth-largest concentration of Puerto Rican residents after Florida, New York, and New Jersey.

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"Who wants to tell these guys there are HALF A MILLION Puerto Ricans living in Battleground PA, whose votes are up for grabs?" former Trump White House aide Alyssa Farah Griffin posted on X.

During the rally, comedian Tony Hinchcliffe joked about the territory being a "floating island of garbage" and made further racist remarks about African Americans and Hispanic immigrants' sex lives.

Other speakers made sexist and crude remarks about Trump's election rival, Vice President Kamala Harris, ensuring that the rhetoric upstaged Trump's big policy rollout, a tax credit for home caregivers.

Trump's campaign – which doesn't typically acknowledge missteps – didn't respond to a request for comment but told US media outlets the garbage joke did not reflect Trump's views, and that Hinchcliffe's set had not been vetted.

In a potent split screen, the scandal blew up as Harris was speaking at Puerto Rican restaurant Freddy & Tony's Restaurant in Philadelphia where she outlined plans to boost the territory's electrical grid.

Moments after Hinchcliffe's remarks, Puerto Rican rapper Bad Bunny – one of the world's top Hispanic celebrities – shared Harris's plan with millions of social media followers.

And Trump supporter and Republican Florida senator Rick Scott, who is in a tight race for reelection against a Latina congresswoman, was among Republican politicians and strategists who voiced anger on X.

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"This joke bombed for a reason. It's not funny and it's not true," he posted, while the Congressional Hispanic Caucus, which has around three dozen members, called the rally "shameful."

Harris, on the campaign trail in Michigan, told reporters the rally highlighted how Trump is "focused, and actually fixated, on his grievances, on himself and on dividing our country."

"If he were elected, on day one he's going to be sitting in the Oval Office, working on his enemies list," she said.

"On day one, if I'm elected president of the United States, which I fully intend to be, I will be working on behalf of the American people on my to-do list."

Speaking to reporters as he voted Monday, President Joe Biden called the scandal "embarrassing" and "beneath a president."