Acting National Arts Endowment chairman Michael Straight said yesterday that he will step down from his job in two weeks - and commented on what he regards as the "politicization" of the arts and humanities by the Carter administration.

Straight, who was the deputy chairman agreed to stay on after Nancy Hanks' second four-year term as chairman ended early this month. The White House has informed a number of persons on Capitol Hill and in the arts establishment that Livingston Biddle is the President's nominee as the new chairman. Confirmation By the Senate would have to come before the closing date of theis session, tentatively scheduled for Oct. 28.

Asked about Straight's decision, a White House spokesman said: "We certainly expect continuity in the top leadership of the Arts Endowment, Because we are counting on an early background clearance in time for the name of the President's nominee could serve as an interim chairman. Straight, in an interview yesterday, said he thinks the prospective Biddle nomination and that of Joseph D. Duffey to be chairman of the Humanities Endowment were affected toomuch by "political considerations." But he made a point of adding that he thought his earlier questioning of Duffey's academic backgroung was "unfair." "He's clearly qualified," said Straight.

Straight stated his belief that the White House staff needs "a strong senior adviser on the arts" along the lines of Leonard Garment in the Nixon administration.

Straight said he regards Joan Mondale, the cVice President's wife, as the strongest figure on the arts at the White House. The administration, Straight said, thinks "Joan should be given a free hand in this area. She does a wonderful job in her traveling, but I think this policy represents a misconception on their part about the problems at the day-to-day level. She needs a strong person there while she's away. Tensions will develop, particularly on money, that must be closely watched. I remember once when Len Garment went back to the Office of Management and Budget for us on a third try, when even we were scared it was a mistake. But he finanally got the money."