White House hopes for an endorsement of President Carter by the United Steelworkers of America fizzled yesterday as the big AFL-CIO affiliate took a temporary vow of neutrality toward presidential politics.

In the face of what one union official called "the heat coming from the White House," the executive board of the 1.2-million-member union decided that "no endorsement of presidential candidates should be made at this time."

The board cited what it called the "preliminary" nature of presidential politicking at this point and said it would steer clear of it in order to devote full attention to contract bargaining that starts early next year.

At one point, Carter labor strategists indicated they expected the USW to join the Labor for Carter/Mondale Committee that got off to a modest start in July with representation from six AFL-CIO unions.

But that was before Sen. Edward M. Kennedy (D-Mass.), who is reported to have strong rank-and-file support in the USW as well as many other unions, indicated earlier this month he may seek the Democratic presidential nomination.

Kennedy's signal has reportedly slowed whatever momentum the labor-for-Carter group had, although officers of the National Education Association are moving, as expected, toward a Carter endorsement.

The Steelworkers' declaration of neutrality follows a decision last week by the United Auto Workers union to abandon its hands-off policy toward the presidential race -- freeing union activists to go their way, which, in most cases, means toward Kennedy.