WASHINGTON (REUTERS) - Donald Trump's White House campaign was in turmoil on Wednesday (Aug 3) after he angered senior Republican Party leaders by criticising a dead soldier's family and refusing to back the re-election campaign of House of Representatives Speaker Paul Ryan.

On Tuesday, Trump denied both Speaker Ryan and Senator John McCain support in their coming primary contests, hitting back at critics in the Republican leadership who have taken him to task for his insistent public dispute with the parents of the soldier, a Muslim US Army captain killed in the Iraq war.

Republican National Committee Chairman Reince Priebus was furious over the failure to endorse Ryan, who is the most senior elected Republican, and over Trump's feud with the Khan family, two Republican sources said. "He feels like a fool," a Republican source familiar with the situation said of Priebus.

More than any other major figure in the Republican establishment, Priebus worked to bring Trump into the party's fold despite the New York businessman's status as an outsider. Trump, who had never previously run for public office, beat 16 rivals to become the Republican presidential nominee for the Nov 8 election.

Ahead of last month's Republican Party Convention, the RNC chairman sought to rally the fractured party behind Trump. Priebus feels burned by Trump's string of self-inflicted wounds and his refusal to observe basic decorum by giving Ryan his support.

But in what appeared to be an effort to soothe ruffled feelings, Trump's vice-presidential running mate, Mike Pence, said on Wednesday he endorsed Ryan as "a strong conservative leader," and was doing so with Trump's blessing.

The Indiana governor told Fox News it takes time to build relationships in politics and that was exactly what Trump and Ryan were doing.

Trump has had a running dispute with the parents of Army Captain Humayun Khan since they took the stage at last week's Democratic National Convention. Khizr and Ghazala Khan cited the sacrifice of their son, who was killed by a car bomb in 2004, and criticised Trump's proposal to combat terrorism by temporarily banning Muslims from entering the United States.

Many Republican leaders, including Ryan and McCain, have criticized Trump's subsequent attacks on the parents. Even his longtime ally, New Jersey Governor Chris Christie, said it was inappropriate to attack the Khans.

Trump, who made his comments about Ryan and McCain in an interview with The Washington Post, shrugged off the backlash.

"There is great unity in my campaign, perhaps greater than ever before. I want to thank everyone for your tremendous support. Beat Crooked H!" he wrote on Twitter early on Wednesday, referring to his Democratic rival, Hillary Clinton.

Campaign manager Paul Manafort told Fox News the campaign was moving in a positive direction, with the candidate himself in control. "The campaign is in very good shape. We are organised. We are moving forward," Manafort said.

However, a Republican source said Manafort was struggling to get the candidate back on message.

RECENT TURMOIL

Trump's always unruly campaign has been hit by disorder in recent days. On Monday, he fired Ed Brookover, a senior adviser hired as a liaison between the campaign and the RNC.

Ryan, who is favored to win next week in his race against primary challenger Paul Nehlen, appeared to be trying to ignore the snub from Trump. The speaker would not have time to meet Trump later this week, when Trump is expected to be campaigning in Wisconsin, Ryan's home state, an aide to Ryan said.

The aide did not indicate whether anyone had requested a get-together of Trump and Ryan, but in response to a query as to whether they might meet to patch things up, the aide told Reuters,

"The speaker has a full schedule - can't back out of previous commitments in the (congressional) district."

A Republican congressional aide said there was deep frustration on Capitol Hill that Trump keeps engaging in "petty spats."

The aide said congressional offices that support Trump got two sets of talking points on Monday from the campaign about the Khan situation but have not heard anything from the campaign about Trump's Ryan comments.

The dispute over Trump's treatment of the Khans was the latest rift in a party frayed by dissent over the candidate.

Late on Tuesday, Meg Whitman, a prominent Republican fundraiser and chief executive of Hewlett Packard Enterprise , endorsed Clinton's White House bid, calling Trump an"authoritarian character" and a threat to democracy.

In better news for Trump, his campaign and the RNC jointly raised US$80 million (S$107 million) for his White House bid in July, the campaign said on Wednesday. That was less than the US$90 million Clinton raised along with the national Democratic Party the same month, but it was a substantial bump from past months.

A former reality TV star with a propensity for free-flowing insults, Trump has won support particularly from white blue-collar workers who feel neglected by the political establishment. His plans have included the ban on Muslims, building a wall along the Mexican border to keep out illegal immigrants and renegotiating trade agreements.

On Tuesday, Democratic President Barack Obama unleashed his strongest attack yet on Trump, calling him unfit for the presidency and asking Republican leaders why they continued to endorse him given their repeated criticisms of his actions.

Opinion polls have shown Clinton benefiting from a boost after her party's convention last week. The RealClearPolitics average of recent national polls put her 4.5 percentage points ahead of Trump, at 46.5 per cent to 42 per cent.