A White House assistant who wrote speeches for Vice-President Agnew during last fall's congressional campaign said yesterday he had expected a greater reaction to last month's invasion of Laos than has so far taken place.

"I figured there would be a considerable reaction," Patrick J. Buchanan, a special assistant to the President, said yesterday, conceding that the possibility of a national outcry similar to the one that followed the invasion of Cambodia last May had been considered by President Nixon when the decision to invade Laos was made last month.

"You realize there's going to be a reaction," Buchanan said. "You consider it and you try to diminish the consequences. No one wants to see any anguish, but the President thought the decision was right and he made it."

Buchanan, a former editorial writer for the St. Louis Globe-Democrat who has been on President Nixon's staff since the pre-campaign of 1966, was a guest speaker at an Institute of Politics seminar yesterday afternoon. His present White House responsibilities include briefing the President before his press conferences and overseeing compilation of a daily news digest.

In an interview last night, Buchanan said that the object of the Laos invasion is "to guarantee the success of American troops left in South Vietnam during the next dry season in October. We expected tough fighting and that's what we're having."

He also defended the tactics employed by the Administration in last fall's congressional campaigns, and said that the results were a "wash," with neither party having won a clear victory. "The object of that campaign was not to bring us together, but to defeat 15 Democratic Senators. On the national level we didas well as we expected, and not as well as we hoped. But any election where we defeat [Senators] Gore, Yarborough, Goodell and Tydings can't be all that bad."

The White House aide said he felt Agnew had been "exceptionally effective" as a campaigner, because "he threw the entire Democratic Party nationally on the defensive." Since the election, he added, the Vice-President's role has changed. "Just as he carried the load in political matters, now he'll have to carry the load in selling programs to the people," he said.

Buchanan said that his visit was not part of any conscious effort by the Administration to improve communications with college students, but noted that such an effort would be "a good idea-as long as you can get a hearing."

"I think we've got a case to make to college students," Buchanan added. "The whole governmental reorganization [proposed by the President in his State of the Union address] is based on principles close to young people."