President Reagan yesterday named former representative Tom Loeffler (R-Tex.) to head a new lobbying effort aimed at persuading Congress to continue military aid to the contras seeking to overthrow the Sandinista government of Nicaragua.

Senior officials also said that White House Communications Director Thomas C. Griscom will soon name an aide to direct "public diplomacy" efforts directed at building support for contra aid. Sally Grimes, a veteran U.S. Information Agency official based in Mexico City, was approached for the job last month but declined.

The new lobbying campaign comes at a time when White House officials are privately acknowledging that they face extremely difficult obstacles in convincing Congress to extend the contra aid program when it expires Oct. 1. Asked who would be picked for the public diplomacy post, a senior official said last week, "First off, you've got to find a masochist."

But these officials said that Reagan remains determined to assist the "Nicaraguan freedom fighters" and will not back away from their cause in the wake of the Iran-contra hearings. The administration is seeking $120 million to aid the contras in fiscal 1988.

Loeffler's appointment was made without fanfare in a written statement issued by the White House press office after the daily briefing. The announcement said that Loeffler "will serve as a coordinator for legislative activities in support of administration initiatives regarding Central America."

White House officials want to take a new approach to the contra lobbying effort because of the unpopularity in Congress of Elliott Abrams, the assistant secretary of state for inter-American affairs. Abrams has acknowledged misleading Congress about a $10 million contribution to the contras from the sultan of Brunei, but Secretary of State George P. Shultz has defended Abrams and refused to fire him.

State Department unhappiness with the appointment of Loeffler appeared to be reflected in the comment yesterday of its spokesman, Charles E. Redman, who said, "The fact remains that Elliott Abrams is the principal administration spokesman and policy person for this particular area of our policy."

Loeffler, 40, left the House in 1986 after four terms to make an unsuccessful bid for the Republican gubernatorial nomination in Texas. In the halcyon days of the administration in 1981-82, Loeffler was considered a key bridge to the southern Democrats known as "Boll Weevils" who gave vital support to Reagan programs.

Loeffler, an attorney who served as a legislative affairs assistant in the Ford administration, also has ties to several Democratic senators. He was recommended to head the new lobbying effort for contra aid by White House deputy chief of staff Kenneth M. Duberstein and chief legislative liaison Will Ball.