NASA officials vowed yesterday that the date of the next space shuttle flight will not be affected by the presidential election or any other political factors.

"The next launch of the space shuttle will be determined by one consideration . . . alone: We will fly when we are ready . . . when it's safe to do so and not before," NASA administrator James C. Fletcher said at a news conference.

"I say that because there's been a lot of talk in Washington that political considerations -- national conventions and the presidential election, for example -- could affect the timing of the shuttle flight. That will simply not be the case."

The shuttle launch date had been set for June 2 but has been postponed until at least late July or August by the failure of a redesigned nozzle part. The White House, Fletcher said, has "made it very clear to me several times that we launch when we're ready, and don't mess around with any political considerations," he added.

Rep. Manuel Lujan Jr. (R-N.M.), ranking Republican on the House Science and Space Committee, said recently, "Experience tells me that . . . nobody in their right mind is going to launch" right before the election. He predicted the flight would be delayed into 1989.

A new date for the next launch will not be set until investigators complete their analysis, probably in another two to three weeks, of what went wrong with a new design for a part known as the outer boot ring in the shuttle solid rocket booster's nozzle, according to Rear Adm. Richard H. Truly, the National Aeronautics and Space Administration's head of space flight.

The part failed a Dec. 23 test firing at Morton Thiokol Inc.'s plant in Utah. The space agency has decided to replace the failed boot ring design with an alternative that was tested successfully last summer.

"If the alternative part passes rigorous testing and our other schedules are met, July 15 is the earliest we could fly," Truly said, adding that a more realistic launch date would be "probably by the middle of August."

The boot ring is part of an assembly that allows the nozzle to swivel and guide the spacecraft.

A factor not fully understood by engineers was the unusually severe swiveling of the nozzle during the December test firing. The replacement design has not yet been subjected to the same degree of stress.

J.R. Thompson, director of Marshall Space Flight Center in Huntsville, Ala., which manages the booster program, said the replacement boot ring design will be thoroughly tested in two remaining full-scale booster firings already scheduled and will be tested "to the extremes the nozzle would be subjected to in flight."

Neither boot ring design had been tested by the time in August when officials had to decide which one to test and when, he said.