Donald Trump's campaign chief has acknowledged conflict inside Trump Tower as anxious Republicans struggled to shift voters' attention to Hillary Clinton's record on foreign policy.
The feud between presidential nominee Mr Trump and House Speaker Paul Ryan continued to overshadow the attacks on Ms Clinton.
Mr Trump refused for another day to endorse the Republican speaker while Mr Ryan publicly declared his support for Mr Trump, but said such endorsements are not "blank cheques".
Mr Ryan pledged to speak out against the businessman's divisive positions if necessary, including his sustained criticism of an American Muslim family whose son, US Army Captain Humayun Khan, was killed in Iraq.
"I don't like doing this," Mr Ryan told a Wisconsin radio station.
"I don't want to do this, but I will do this because I feel I have to in order to defend Republicans, and our principles, so that people don't make the mistake of thinking we think like that."
Campaign chairman Paul Manafort insisted Mr Trump would work with Mr Ryan if elected, but he conceded the endorsement question had sparked tension inside Trump's New York campaign headquarters.
The day before, vice presidential nominee Mike Pence broke with Mr Trump and endorsed Mr Ryan.
"There's a conflict within the Trump campaign," Mr Manafort told ABC's Good Morning America.
''We've sort of had a rule of not getting involved in primaries because it's usually not a good situation for the presidential candidate."
As Republican infighting dominated the 2016 presidential race for another day, Mr Trump and his Republican allies lashed out at the Democratic presidential nominee's foreign policy record.
They contended that Ms Clinton was responsible for negotiations that led to a 400 million US dollar (£304 million) payment to Iran earlier in the year.
Mr Trump has described the money as a ransom payment for four Americans detained in Iran, although the payment - plus 1.3 billion dollars (£990 million) in interest to be paid later - is a separate issue from the Iran nuclear deal that Ms Clinton initiated.
As Mr Trump used the issue to assail Ms Clinton, he faced new questions about his Wednesday description of "a military tape" he suggested was taken by Iranian forces removing bags of money from a plane. It is unclear what footage Trump was referring to.
Several senior US officials involved in the Iran negotiations said they were not aware of any such video. Mr Trump's campaign said the GOP nominee had not seen a tape as part of any security briefing.
In an email, a campaign spokeswoman said Mr Trump was simply referring to video aired on television this week, although no such footage of payment to Iranians was shown.
At the same time, Ms Clinton criticised Mr Trump for outsourcing at his companies the very jobs he is promising to create back home.
"Everything he's made he's made somewhere else," Ms Clinton said as she toured a Las Vegas electric manufacturer on Thursday afternoon.
"I've met people who were destroyed by Donald Trump, so take a look at what he's done, not what he says."
Meanwhile, former New York Mayor Rudy Giuliani dismissed reports he was planning an "intervention" with top Trump allies, including GOP chairman Reince Priebus and former House Speaker Newt Gingrich, to urge Mr Trump to abandon tactics that have triggered sinking poll numbers and low morale.
"I meet with Donald Trump all the time," Mr Giuliani told the Fox Business Network, suggesting that Gingrich used the word "intervention" in a recent memo.
"He is a new candidate," Mr Giuliani said of Mr Trump. "That adds a little bit of - more of a learning curve, than would normally be the case."
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