(Adds background on Bannon, Manafort reports)

By Caren Bohan

WASHINGTON, Aug 17 (Reuters) - U.S. Republican presidential candidate Donald Trump moved to overhaul his troubled campaign on Wednesday, hiring the head of conservative website Breitbart News as chief executive officer and promoting a seasoned political operative to a senior role.

Stephen Bannon, the combative executive chairman of Breitbart, was named CEO, the Trump campaign said in a statement. It also said Kellyanne Conway, who has been an adviser, will take on the role of campaign manager.

The leadership changes come as opinion polls show Trump, a wealthy New York businessman, falling behind Democratic candidate Hillary Clinton in the race for president.

Trump, who has never held elected office, has been facing a barrage of criticism from Republicans over his freewheeling campaign style and his refusal to stick to a policy message.

He came under fire for his prolonged feud with the Muslim family of a U.S. soldier who was killed in the line of duty in Iraq and for his unfounded accusation that President Barack Obama and Hillary Clinton were co-founders of the Islamic State militant group. Trump later backed off the comments about Islamic State.

The campaign's announcement quotes Trump as saying he was "committed to doing whatever it takes to win" the election on Nov. 8. The campaign also said it will make its first major television commercial purchase later this week.

The staff changes, first reported in the Wall Street Journal, mark the second time in two months Trump has shifted the leadership in his campaign. In June, he fired longtime aide Corey Lewandowski as campaign manager and handed more power to senior campaign aide Paul Manafort.

The statement from the Trump campaign said Manafort would remain as campaign chairman and chief strategist.

Adding to Trump's woes this week was a New York Times report that Manafort's name was on secret ledgers showing cash payments designated to him of more than $12 million from a Ukrainian political party with close ties to Russia. Manafort denied any impropriety in a statement on Monday.

Bannon, a former Goldman Sachs GS.N banker who also served in the U.S. Navy, came under criticism as not supporting Michelle Fields, a female reporter who said she was grabbed and bruised by Lewandowski at a March campaign event in Florida. Lewandowski, then Trump's campaign manager, was charged with battery but the charge was later dropped.

Ben Shapiro, a Breitbart editor who resigned from the organization along with Fields, called Bannon a bully who sold out to "another bully, Donald Trump," to protect Trump's man.

"He has shaped the company into Trump's personal Pravda, to the extent that he abandoned and undercut his own reporter," Shapiro said in a statement at the time. (Reporting by Caren Bohan; Editing by Lisa Von Ahn and Jeffrey Benkoe)

Keywords: USA ELECTION/