U.S. Republican presidential nominee Donald Trump has shaken up his campaign staff amid sliding poll numbers and signs of disarray, U.S. media reported early on Wednesday.

Trump made senior advisor Kellyanne Conway his campaign manager, and Breitbart News executive chairman Stephen Bannon has been brought on as the campaign's chief executive, the New York Times reported citing Conway.

Paul Manafort would remain as campaign chair, it said. The Washington Post cited campaign aides as saying that while Trump respected Manafort, he felt "'boxed in' and 'controlled' by people "who barely knew him".

Manafort was forced to deny any impropriety this week after the New York Times reported his 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 has denied impropriety.

Manafort took on the role in March after Trump fired his predecessor Corey Lewandowski.

The shakeup comes as opinion polls show Trump, a wealthy New York businessman who has never held elected office, falling behind Democratic candidate Hillary Clinton in the race for the Nov. 8 election.

The Wall Street Journal said on Monday in an editorial that Trump should fix his stumbling White House campaign by Labor Day on Sept. 5 or step down.

(Reporting by Curtis Skinner in San Francisco; Editing by Raissa Kasolowsky)