As Republicans announced they were moving forward with an impeachment inquiry, fresh evidence emerged this week casting doubt on some of the key claims.
Thomas Sobocinski, an F.B.I. agent, told House investigators that the prosecutor overseeing the investigation into the president’s son never said he did not have full authority to pursue charges.
An examination of confidential correspondence and interviews with those close to Mr. Biden and lawyers involved in the case show how the deal ultimately fell apart amid schisms and withering external pressures.
Empower Oversight, a group of lawyers including former aides to Senator Charles E. Grassley, Republican of Iowa, is part of a constellation of groups facilitating G.O.P. investigations in Congress.