The stepped-up role for the convention manager, Paul Manafort, a veteran of floor fights whose presence on Mr. Trump’s campaign has created anxiety among other top aides, was intended in part to quash reports of infighting and concerns about an organization whose performance has been lackluster at best in a recent string of nominating contests.

The expansion was announced a day after a lengthy and, at times, tense meeting that included Mr. Trump, Mr. Manafort and the campaign manager, Corey Lewandowski, who has been the focus of intense scrutiny after Mr. Trump’s lopsided loss in Wisconsin on Tuesday night, difficulty recruiting allies for some state conventions and Mr. Lewandowski’s recent arrest on battery charges.

“He had a different campaign model and his campaign model was precipitated on the concept of mass communication, establishing who he was, and working what I consider to be a very historic approach to communication with the public, so winning was what mattered. It didn’t matter how big” the campaign was at first, Mr. Manafort said in a telephone interview.

“The campaign has now gotten to the point where how you win and where you win does matter and then protecting what you won and that’s why I’m involved in everything from what’s happened to what will be happening,” he said.