Republican party nominee Donald Trump has said that he would slash taxes, block onerous financial regulations and unleash the energy sector as he pledged to "jump start America" with a new economic plan if elected United States president.

Separately, as the White House nominee attempted to reset his campaign, dozens of top Republican security officials published a joint letter in The New York Times warning that he would be "the most reckless president in American history." 

The property tycoon and former reality TV star unveiled his proposals in the economically depressed city of Detroit as he attempted to focus on policies that drew a sharp contrast with rival Hillary Clinton.

"We are in a competition with the world, and I want America to win," Trump told an audience at the Detroit Economic Club, as he highlighted "disastrous" policies that he said have snuffed out US jobs in the nearly eight years of Barack Obama's presidency.

"I want to jump start America. It can be done, and it won't even be that hard," he said.

Trump laid out a series of policies to revitalise a limping economic engine, including a sharp reduction of corporate tax from 35 percent to 15 percent, something he floated back in September as a way to lure back US corporations that relocated abroad.

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He said he would also set a 10 percent tax on the "trillions of dollars from American businesses that is now parked overseas" and gets repatriated into the US.

Trump said he wanted to "cut regulations massively", a move he said would lift what he called an "anchor" weighing down small businesses, something Republicans have sought for years during Obama's tenure.

The 70-year-old billionaire also proposed repealing the estate tax, a controversial levy on estates of the deceased valued at more than $5.45 million.

"American workers have paid taxes their whole lives, and they should not be taxed again at death - it's just plain wrong," Trump said.

The speech was interrupted more than a dozen times by protesters, who were escorted out by security.

'Trickle down economics?' 

As he tried to pivot away from recent controversy about his candidacy, Trump portrayed Clinton as the "nominee from yesterday."

"There will be no change under Hillary Clinton - only four more years of Obama," he said. "But we are going to look boldly into the future." 

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Clinton, a 68-year-old former secretary of state and senator, has enjoyed a strong bounce in polls since securing the Democratic nomination last month, the first time a woman has become the nominee of a major party.

A Monmouth University Poll released on Monday showed her ahead of Trump by double digits, 46 percent to 34 percent - a dramatic increase from the three-point lead she held days before the Republican convention.

Clinton used her rally in St Petersburg, Florida to savage Trump's economic plan as an effort to "repackage trickle down economics".

"His tax plans will give super big tax breaks to large corporations and the really wealthy," she said.

"I am not going to raise taxes on the middle class, but with your help we are going to raise it on the wealthy, because that's where the money is."

She cited a study by Mark Zandi, a former economic adviser to Republican Senator John McCain, which predicted that under Trump's plan, the economy would shed 3.4 million jobs and tumble into recession.

"Economists left, right, in the middle all say the same things, that Trump's policies would throw us into a recession," she said.

'A dangerous president'

Also on Monday, some 50 senior Republican national security officials issued a stinging rejection of Trump, warning that, if elected, he would be the most dangerous president in American history. 

The group, some of whom had already announced that they would not vote for him, included former homeland security chiefs, intelligence directors, senior presidential advisors and a former US trade representative.

They served under Republican presidents from Richard Nixon to George W Bush.

The signatories include Tom Ridge and Michael Chertoff, the first and second homeland security secretaries under president George W Bush, former director of National Intelligence John Negroponte and Bush-era CIA director Michael Hayden.

"We are convinced that he would be a dangerous president and would put at risk our country's national security and wellbeing," they wrote in an open letter.

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While they did not say they would vote for Clinton, expressing "doubts" about her, they stated: "None of us will vote for Donald Trump."

He has shown no willingness to learn about foreign affairs or national security threats, "acts impetuously" and lacks self control, the letter said.

"We are convinced that in the Oval Office, he would be the most reckless president in American history."

Source: Agencies