CHARLOTTE: For the first time since declaring his presidential run, Republican Donald Trump acknowledged that his caustic comments may have caused people pain, saying that he regrets some of what he's said "in the heat of debate".

A day after announcing a campaign shake-up and as he trails in the polls, the GOP nominee said that he recognised that his comments — which have angered minorities and alienated large swaths of the general election electorate — may have been ill-advised.

"Sometimes in the heat of debate and speaking on a multitude of issues, you don't choose the right words or you say the wrong thing. I have done that," the GOP nominee, reading from prepared text, said at a rally in Charlotte, NC. "And believe it or not, I regret it — and I do regret it — particularly where it may have caused personal pain."

He added that, "Too much is at stake for us to be consumed with these issues." As the crowd cheered, Trump pledged to "always tell you the truth".

The remarks came as Trump was trying to rescue a campaign that has struggled since the Democratic and Republican nominating conventions from a series of self-created distractions. Early Wednesday, Trump announced that he was overhauling his operation, bringing in a new chief executive and appointing a new campaign manager.

Rarely do presidential campaigns wait to advertise, or undergo such leadership tumult, at such a late stage of the general election.

Yet Trump has struggled badly in recent weeks to offer voters a consistent message, overshadowing formal policy speeches with a steady stream of self-created controversies, including a public feud with an American Muslim family whose son was killed while serving in the US military in Iraq.

Trump's decision to tap Stephen Bannon, a combative conservative media executive, as his new campaign chief, suggested to some that he planned to double down on the playbook he used in the primary, playing to his angry rally crowds and bouncing from one controversy to the next.

Instead, a new Trump emerged on Thursday: a less combative, more inclusive candidate who said he was running to be the "voice for every forgotten part of this country that has been waiting and hoping for a better future” and for those who “don't hear anyone speaking for them".

Earlier Thursday, Trump moved to invest nearly $5 million in battleground state advertising to address daunting challenges in the states that will make or break his White House ambitions.

The New York businessman's campaign reserved television ad space over the coming 10 days in Florida, North Carolina, Ohio and Pennsylvania, according to Kantar Media's political ad tracker. While Democrat Hillary Clinton has spent more than $75mn on advertising in 10 states since locking up her party's nomination, Trump's new investment marks his first of the general election season.

Election Day is 81 days away, with early voting in the first states set to begin in five weeks.

In his remarks, Trump struck a new, inclusive tone and tried to appeal directly to non-white voters, shown by polls to an overwhelmingly unfavourable view of the candidate.

"I will not rest until children of every colour in this country are fully included in the American Dream," Trump told his audience, again accusing Democratic Hillary Clinton of "bigotry".

Clinton, he claimed, "sees communities of colour only as votes and not as human beings worthy of a better future".

He urged African-American voters to give him a chance, saying: "What do you have to lose by trying something new?"

Clinton's campaign, meanwhile, brushed the speech off as just words he read from a teleprompter.

"Donald Trump literally started his campaign by insulting people. He has continued to do so through each of the 428 days from then until now, without shame or regret," said spokeswoman Christina Reynolds in a statement.

"We learned tonight that his speechwriter and teleprompter knows he has much for which he should apologise. But that apology tonight is simply a well-written phrase until he tells us which of his many offensive, bullying and divisive comments he regrets and changes his tune altogether," she said.

It remains to be seen whether Trump's reboot comes too late, and whether he has the discipline to maintain it.

Trump now trails Clinton in preference polls of most key battleground states. And his party leaders, even at the Republican National Committee, have already conceded they may divert resources away from the presidential contest in favour of vulnerable Senate and House candidates if things don't improve.

But Trump supporters largely accepted the change of tone, even if some saw it as unnecessary.

"It takes a lot of strength to say, 'I'm sorry', to admit — not that he was wrong, but he wished he hadn't done it," said Cindy Ammons, 70, a Trump supporter from Spindale, North Carolina,

"I think he's evolving," she said.