Convicted Watergate conspirator James W. McCord Jr. testified before the Senate hearings today that when he agreed to wiretap the Democratic headquarters he believed the mission had the full approval of the attorney general, John N. Mitchell.

McCord also said, under close questioning by Senator Lowell P. Weicker Jr. (R-Conn.), that the Justice Department provided him with detailed information on the political opposition while he was chief security officer for the Committee to Re-Elect the President (CRP).

McCord, his voice hoarse and cracking, told the national television and radio audience that he had received repeated offers of executive clemency, ostensibly with the approval of President Nixon, from a former White House aide who was his close personal friend.

He testified that John Caulfield, a White House aide who later moved to the Department of Transportation, applied "political pressure" to him during the Watergate trial to convince him to remain silent. He said that the pressure included warnings that the fate of the Presidency might hinge on his decision.

Hearsay Evidence

During four-and-a-half hours of testimony in the morning and afternoon, Senator Sam Ervin Jr. (D-N.C.), chairman of the select committee, continually emphasized that most of McCord's testimony involved hearsay evidence--information obtained second hand--which was admissible only to establish McCord's motives for participating in the Watergate conspiracy.

McCord testified that G. Gordon Liddy, then chief counsel for the CRP, approached him in early 1972 with a detailed plan for political espionage, photography and wiretapping to be conducted against Democratic presidential candidates and the Democratic Party.

Liddy told him that the plan was concocted in the office of attorney general Mitchell in meetings between Liddy, Mitchell, campaign deputy Jeb S. Magruder and presidential counsel John W. Dean III, McCord told the astonished Senators.

McCord said that he had reservations about the legality of the proposed operation at first, but that he was told by Liddy that the attorney general had conferred with his superior, presumably Nixon, in formulating the plan.

A Burst of Laughter

McCord conceded under questioning from committee co-chairman Howard H. Baker (R-Tenn.) that he had never had direct contact with Mitchell, Magruder or Dean in connection with the wiretap plan. When asked what Mitchell had called him in their campaign contacts, McCord drew a burst of laughter by saying, "Before or after June 17?"

McCord and four other men were apprehended by Washington police on the sixth floor of the Watergate office Building in the early morning hours of June 17, 1972. The five men, plus Liddy and E. Howard Hunt, a White House aide, were convicted in January of conspiracy and burglary in connection with the incident.

McCord testified that Liddy arranged to pay him $3000 a month for his political espionage activities and provided him a budget of $76,000 for the purchase of electronic surveillance equipment and burglary tools.

Liddy paid him in cash, McCord said, and Liddy and Hunt handled the recruiting of additional agents for the break-in squad. After the June 17 arrest, McCord testified, Hunt and his wife, who is now deceased, continued to provide him with $3000 in cash per month plus $25,000 in legal fees.

McCord's responses to Weicker's questions about information he had received from the Justice Department apparently came as a shock to the Senate panel which had already interviewed the former CIA agent in closed executive session.

McCord told the committee that the Internal Security Division of the Justice Department--then headed by James Mardian, who later became a campaign coordinator for CRP--provided him with information on the movements and activities of the political opposition from January to June of 1972. He said this intelligence related particularly to political demonstrations or violence.

McCord also said that similar information about the Republicans was made available to the Democratic Party, although NBC News reported late this afternoon that Frank Mankiewicz, campaign director for Senator George McGovern (D-S.D.), denied that his staff had ever seen such information.

Both Republican and Democratic Senators on the committee expressed disbelief at McCord's testimony that he had believed Mitchell's authorization made the wiretapping and burglary legal. Senator Daniel K. Inouye (D-Hawaii) repeatedly demanded whether McCord though that O'Brien, McGovern or Muskie represented a threat to national security, or in his words, "a violent threat to overthrow the government."