The Ervin Committee hearings will resume today as former Attorney General John N. Mitchell testifies about his role in the ever-widening Watergate scandal. Meanwhile, the controversy surrounding President Nixon's silence in the affair broadened yesterday as bipartisan criticism of Nixon's refusal to appear before the Committee erupted.

On Saturday President Nixon sent a personal letter to committee chairman Sam J. Ervin (D-N.C.) in which he invoked separation of powers in affirming his refusal to appear before the body under any conditions. He also stated that access to all Presidential papers was out of the question, even if subpoenaed by the Ervin Committee.

Senator Edward Gurney (R-Fla.), frequently regarded as a White House stalwart, joined Senator Robert D. Byrd (D-W.V.) and Gov. Daniel Evans (R-Wash.) in recommending that President Nixon appear before a news conference to publicly explain any possible White House involvement in the affair.

The expanded bipartisan effort to convince the President to publicly discuss any part he may have played in the Watergate scandal has taken several forms. Many Republicans and Democrats have united in recommending that the president disclose his knowledge of the affair at a press conference, before the Ervin Committee and by release of all White House files concerning the scandal.

When Mitchell appears before the Ervin Committee tomorrow he is expected to answer charges made in earlier testimony that he participated in the planning and cover-up of the June 17, 1972 Watergate break-in. Mitchell has repeatedly denied any role in the affair. Mitchell's attorneys have said that their client will not implicate the president in the scandal.

Mitchell currently is facing conspiracy and perjury charges handed down by a New York Federal Grand Jury.