Donald J. Trump will return to Washington on Thursday for a string of meetings with members of the House and Senate Republican caucuses, some of whom have held out their support.
Among members of Congress, many from districts populated by Trump supporters, backing for the New York developer is more easily detected. But among members of the Senate, who are hoping to hang onto the Republican majority and their seats, support has been less forthcoming.
Mr. Trump will face a question-and-answer session with House members. A number of senators have indicated they have scheduling conflicts, and their participation is unclear.
Two, Senator Bob Corker of Tennessee and Senator Joni Ernst of Iowa, said on Wednesday that they were not seeking to serve as Mr. Trump’s running mate, though they had initially been considered.
Mr. Trump might find common cause with them in celebrating the elements of the news conference held by the F.B.I. director, James B. Comey, on Tuesday, in which he said that Hillary Clinton had been “extremely careless” with her private email server as secretary of state, and he said bluntly that she should have known better.
The race is still fairly close in national polls, a fact that Mr. Trump’s allies and even Republicans who do not support him point to. But the presumptive Republican nominee has overshadowed what should be one of the toughest weeks of Mrs. Clinton’s campaign with a series of self-inflicted controversies. How he sells his case to members of his party will be telling of how Mr. Trump views the state of the race.