Republican candidate Donald Trump has called Democratic rival Hillary Clinton "the devil", in the face of continuing fire from within his own party.
Speaking at a rally in a high school gym in Pennsylvania, Mr Trump attacked Bernie Sanders for capitulating to Mrs Clinton in the Democratic race.
"He made a deal with the devil," the tycoon said. "She's the devil."
Democrats and Republicans alike have condemned Mr Trump for his remarks about a US Muslim soldier's parents.
Former Republican candidate John McCain became the latest senior Republican to criticise Mr Trump for his attack on the parents of US Army Capt Humayun Khan, who was killed by a car bomb in 2004 in Iraq, at the age of 27.
Senator McCain, a veteran of the Vietnam War, said in a strongly worded statement that Mr Trump did not have "unfettered licence to defame the best among us".
The soldier's parents, Khizr Khan and his wife Ghazala, told the BBC it was time to stand up to Mr Trump but he accused them of "viciously" attacking him.
Mr Trump had caused controversy by suggesting Ghazala Khan had been prevented from speaking alongside her husband at the Democratic convention last week.
The Republican candidate had already described the decision of Mr Sanders to back Mrs Clinton at the convention as a "deal with the devil".
However, his remarks on Monday evening in the gym in Mechanicsburg, Pennsylvania, were the first time he went so far as to specifically equate Mrs Clinton with the devil.
In another development, American billionaire businessman Warren Buffett challenged Mr Trump to release his tax returns.
Mr Trump has said that they cannot be made public until the financial authorities have completed an audit.
But Mr Buffett said there were no rules against showing tax returns and allowing people to ask questions about them. Speaking at a rally in support of Mrs Clinton, Mr Buffett said he was under audit as well, adding he was prepared to meet Mr Trump "any place, any time" to go over each other's tax returns.
Reaction to Mr Trump's comments
US President Barack Obama made an implicit dig at Mr Trump, saying: "No-one has given more for our freedom and our security than our Gold Star families."
South Carolina Senator Lindsey Graham, Mr Trump's former primary opponent, said: "'Unacceptable doesn't even begin to describe it."
House Speaker Paul Ryan condemned any criticism of Muslim-Americans who serve their country, but avoided mentioning Mr Trump.
Ohio Governor John Kasich tweeted: "There's only one way to talk about Gold Star parents: with honour and respect."
Former Florida Governor Jeb Bush called Trump's remarks "incredibly disrespectful".
Veterans of Foreign Wars President Brian Duffy: "Election year or not, the VFW will not tolerate anyone berating a Gold Star family member".
Alabama Senator Jeff Sessions, a Trump supporter, defended Mr Trump's remarks, telling CNN: "His interview was not unkind. It was respectful. It did express condolences to the family for their loss."
How many Muslims are there in the US?
Pew Research Center's 2014 Religious Landscape Study found that 0.9% of US adults identify themselves as Muslims.
A 2011 survey of Muslim Americans, estimated that there were 1.8 million Muslim adults (and 2.75 million Muslims of all ages) in the country. That survey also found that a majority of US Muslims (63%) are immigrants.
Demographic projections estimate that Muslims will make up 2.1% of the US population by the year 2050, surpassing people who identify as Jewish on the basis of religion as the second-largest faith group in the country (not including people who say they have no religion).
A Pew Research Center report estimated that the Muslim share of immigrants granted permanent residency status (green cards) increased from about 5% in 1992 to roughly 10% in 2012, representing about 100,000 immigrants in that year reports the BBC.