Mr. Ryan would dedicate most evenings to dinners with senators and House members of both parties, aides said, as he steps into the role Mr. Romney promised: architect of a Romney administration’s drive to enact a budget that shrinks the government and overhauls programs like Medicare…

The prospect of a deeply engaged vice president was described in interviews with campaign aides, close House colleagues and the few times Mr. Ryan has discussed his potential future job. Asked by a reporter last month if he expected the kind of broad responsibility for the economy that Mr. Cheney held for national security — as an aide suggested — Mr. Ryan said, “I do.”

“A large reason he was chosen was to help Romney govern,” an adviser to the campaign said. “Paul’s going to focus on being a partner.”…

Before Mr. Ryan accepted the nomination, aides said, he had extensive conversations about his position with Mr. Romney, who assured him he would play a guiding role on fiscal and economic matters.