Donald Trump is becoming more isolated within the Republican Party, with the presidential candidate calling its members 'disloyal' and 'foul-mouthed'.
Senator John McCain is the latest in a string of Republicans to pull their support for Mr Trump.
Mr McCain withdrew his support for the businessman and reality TV star over Mr Trump's views on women.
"When Mr. Trump attacks women and demeans the women in our nation and in our society, that is a point where I just have to part company," he said.
"It's not pleasant for me to renounce the nominee of our party. But... I have daughters. They cannot be degraded and demeaned in that fashion."
Mr Trump hit back in a series of tweets on Wednesday morning (NZ time) calling Mr McCain "foul mouthed" and claiming he "begged" for his support during the senator's primary.
He said disloyal Republicans are worse than "crooked Hillary", and that it was "nice that the shackles have been taken off me".
He also said House Speaker Paul Ryan, who refuses to defend Mr Trump, is a "weak and ineffective leader".
Mr Ryan has told congressional Republicans he will put his energy into preserving party majorities in Congress, all but conceding that Democratic candidate Hillary Clinton would likely win the White House in the November 8 election.
The move angered some Trump supporters, although Mr Ryan said he would not withdraw his endorsement of the New York businessman.
Meanwhile, whistleblowing platform Wikileaks has released a third wave of personal emails from Ms Clinton's campaign, revealing her team had a 71-page document of prepared 'hits' against Democratic rival Bernie Sanders.
In the document, the campaign's research director, Tony Carrk, outlines key areas where Ms Clinton could attack Mr Sanders, including immigration, LGBT rights and gun control.
"Attached are some hits that could either be written or deployed during the next debate on Sanders," wrote Mr Carrk on October 28, 2015, about two weeks after Mr Sanders was reported by US media as the better candidate in the initial Democratic debate.
Mr Trump condemned Ms Clinton as "disgraceful" following the leaks, but the Democratic nominee has pointed to the Obama administration's accusations that Russia is using the email dumps to influence the November 8 election.
Mr Trump, whose campaign has been marked by controversies over both his policies and brash style, has slipped further behind Clinton in opinion polls.
Many Republicans are worried his chaotic campaign could hurt their chances of holding majorities in the House of Representatives and Senate in next month's election, and will inflict long-term damage on the party.
In an extraordinary party revolt, nearly half of all 331 incumbent Republican senators, House members and governors have condemned Trump's lewd remarks on a recently released video from 2005.
Roughly one in 10 Republican senators have called for him to drop out of the race, a Reuters review of official statements and local news coverage indicates.