Patrick J. Buchanan, the fast-rising White House director of communications, acquired new authority yesterday when White House chief of staff Donald T. Regan decided to give him control of the Office of Public Liaison, senior administration officials said.

The conservative former columnist is scheduled to take over within a few weeks when the present director of the office, Faith Ryan Whittlesey, leaves for a second tour of duty as ambassador to Switzerland.

White House sources said Buchanan also stood a good chance of being awarded control of the out-of-town press operation, now under deputy chief of staff Michael K. Deaver, after Deaver leaves to join a public relations firm in May. But they said no final decision had been made.

"Pat is becoming the real power in the White House, second only to Regan," one official said. "He knows how to move, and he's moving very fast."

The Office of Public Liaison will give Buchanan what he now lacks -- a substantial contingent of subordinates who can carry his views of presidential policies to a variety of constituent groups.

In winning control of the public liaison office, Buchanan prevailed over another Regan deputy, political assistant Edward J. Rollins.

Last week Rollins and Buchanan reportedly agreed on a plan that would divide supervisory authority over the 36 employes in the public liaison office between them.

But Regan, for reasons that have not been explained, rejected the proposal and gave command of the entire operation to Buchanan, officials said.

If Buchanan also gains control of out-of-town press operations, it would bring him into potential conflict with another senior White House official, deputy press secretary and spokesman Larry Speakes.

Presently, the responsibilities of Buchanan and Speakes are rigidly separated. Speakes has been the spokesman, reporting directly to the chief of staff. Buchanan has been what Regan calls "a communications planner," in charge of long-distance strategy.

White House officials say the bureaucratic struggle for power within the White House has policy implications. These officials say that in inner councils of the administration Buchanan has favored a hard, confrontational line in dealing with Congress and in advocating President Reagan's policies.

His principal impact so far has been reflected in Reagan speeches, which have taken a more confrontational tone on a variety of economic and foreign policy issues.

Regan reportedly had promised the out-of-town speech operation to Speakes but changed his mind. Although Buchanan, a former speechwriter for President Richard M. Nixon, is theoretically on a level with three other deputies in the White House, the new decisions appear to make him first among equals.

"Pat's clearly No. 2," one official said. "The way he's going, he may be No. 1 someday."