“Bringing Manafort in was a step in the right direction,” said Rudolph W. Giuliani, the former New York mayor and a friend of Mr. Trump. Mr. Giuliani has said he will vote for Mr. Trump but has no plans to make a formal endorsement. But he added that Trump must “do about 10 more of those” steps to get up to speed.

Mr. Lewandowski and Michael Glassner, the deputy campaign manager, have been described by people in the campaign as viewing Mr. Manafort, who is working on a volunteer basis, as a threat. Mr. Manafort is the former business partner and longtime friend of Roger Stone, an informal adviser to Mr. Trump who left the campaign last summer after clashes with Mr. Lewandowski. Mr. Stone introduced Mr. Manafort to Mr. Trump at the 1988 Republican National Convention, as Mr. Stone was squiring the New Yorker around, though the two had known each other since the early 1980s, Mr. Manafort said.

Mr. Manafort’s team is expected to include several aides from the presidential campaigns of both Gerald R. Ford and Ronald Reagan — the two men who battled over delegates at the last contested convention, in 1976 — as well as a few operatives from the orbit of Ron Paul, a Libertarian and a three-time presidential candidate known for his organizing prowess. It will also include two people hired by Mr. Lewandowski – Barry Bennett and Ed Brookover – to work on delegate-wrangling.

Mr. Manafort said he was also hiring people to handle everything from delegate selection to the rules committee to managing a floor operation. “It includes operatives from every past presidential campaign over the last 20 years,” he said.

In an interview, Mr. Lewandowski dismissed questions about tensions as being overblown, and added that Mr. Manafort will also oversee a new Washington office. His purview will not only include delegate oversight, but congressional outreach and working with the Republican National Committee.

“Michael and I continue to run the day-to-day operations of this campaign including the overall strategy” for securing the nomination, Mr. Lewandowski said. He added that Mr. Trump is planning to give a series of policy speeches, and will be laser-focused on New York’s April 19 primary.