With the chairmanship of the National Endowment for the Humanities vacant for five months, President Carter has stepped in for a hard look at both the arts and humanitities endowments and who will run them during his administration.

The President called two key "culture" congressmen - Rep. John Brademas (D-Ind.) and Sen. Claiborne Pell (D-R.I.) - to the White House this week for a half-hour discussion on the topic.

It came amid rumor-a-week curiosity over who would get the humanities post, and offered indications that the White House still is exploring its cultural options. One of the options - merger of the two endowments - could be expected to run into Hill opposition.

The White House would say only that the discussion was a "broad-ranging review of the cultural endowments" and didn't get down to names of candidates for the vacant chairmanship at the humanities endowment.

If they weren't mentioned at the White House meeting on Wednesday, there has been no lack of names in circulation. The job, which pays $52,500, has been vacant since Ronald Berman resigned Jan. 20. Humanities has a budget of $130 million.

At least 80 names have been considered by the President's selection committee, Barry Jagoda, special assistant to the President, said yesterday. But no front-runner has emerged, he stressed, and the search goes on.

Among the rumor favorites have been Charles Blitzer, Smithsonian assistant secretary for history and art, and James H. Billington, director of the Woodrow Wilson International Center for Scholars at Smithsonian.

Blitzer has been asked for his views on the humanities post and is known to have talked to the President briefly. Billington has the backings of Brademas, who is the House Majority Whip and chairman of the House education subcommittee on the arts.

Pell, who wrote and sponsored, the legislation setting up the humanities endowment 12 years ago, also looks favorably on Billington after the withdrawal of his No. 1 candidate, Robert Lumiansky, president of the American COuncil of Learned Socities and head of the United Chapters of Phi Beta Kappa.

Two of the other names that have surfaced have been academic colleagues of Lumiansky. One - Otis Singletary Jr., president of the University of Kentucky - turned down the offer of the humanities post.

Singletary citied bothe institutional and personal concerns. It was presumed that the endowment chairman's salary was less than the university presidency pays.

Frank Vandiver, provost at Rice University in Houston and now the advisory vice-chairman of the National Council of the Humanitites, also has been mentioned as a prospect. Another name has been that of Warren Bennis, president of the University of Cincinnati.

After the meeting with President, Pell said the call to the White House was evidence of Carter's interest in cultural matters. Among those attending were two members of the White House staff, Jagoda and Stuart Eizenstat, White House domestic adviser.

Hill sources said the President and congressmen talked about the "mission" of the humanities endowment and the special qualifications needed for a new chairman.

"You need someone who can be both the lonely scholar and the mass communicator to get the fruits of research to the general public," one observer said.

The president rumors about candidates for the humanities chairmanship points up the status and prestige achieved endowments. Each now administers a large budget and the endowment heads have considerable public exposure.

In recent years, the humanities have carried on as a sort of Cinderella sister in the shadow of the more glamorous arts endowment, with its performing arts and growing audiences.

Pell, in blocking the reappointment of Republican Berman to a four-year term last year, made it clear that he thought the humanities endowment has been practising "elitism." The Rhode Island senator wanted more money funneled through state agencies to reach smaller institutions beyond the Eastern Establishment.

The humanities is charged with both encouraging scholarly research and spreading that knowledge. It has come under criticism for not doing enougn to reach the general public, although has been moved into television with such shows as "The Adams Chronicles" and "The American Short Story."

With that move, however, there are some who see a blurring in the line of demarcation between the arts and humanities functions. It, therefore, seems a natural that Carter, a President strong on reorganization, might look into a possible merger.

Many prospects have been angling for the plum appointment as chairman of the high-profile Arts Endowment. Nancy Hanks, a Republican appointee with admirers even among Democrats on the Hill, will come to the end of her second four-year term in October.