In public, Donald Trump is standing behind embattled campaign manager Corey Lewandowski as he faces battery charges for grabbing a reporter. But behind the scenes, Lewandowski's role in the campaign is shrinking. In early March, Lewandowski ceded authority over many hiring decisions to a lower-ranking staffer. In recent days, the campaign’s press office has been overruling his decisions about issuing credentials for campaign events. Going forward, Trump’s just-named convention manager, Paul Manafort, is expected to take a leading role not just in the selection of delegates, but in the remaining primaries themselves, according to three people on or close to...