Sen. John Tower, the Texas Republican who earlier this week was widely touted as a front-runner to become the new secretary of defense, has been told he is not likely to get that job, according to sources on the transition team of President-elect Ronald Reagan.

Tower, the sources say, is prepared to be "a good soldier" about the decision but is said to be disappointed and will be even more so if another lawmaker is given the Pentagon post.

Details on why the decision is going against Tower are sparse, apparently because the subject is sensitive within Republican circles.

When Tower's name surfaced as a leading contender for the Pentagon job, some Republicans felt that moving him from Capitol Hill across the Potomac was taking an unecessary risk with the newly won Republican majority in the Senate.

Tower, it was pointed out, could be succeeded in the Senate by another Republican to be appointed by Texas' GOP governor, William Clements. But according to state law, a special election would have to be held within 90 days, making it conceivable that the GOP seat might fall to the Democrats and needlessly reduce the Republicans' newly won 53-to-47 majority in the Senate.

Sources say, however, that it was not the possible loss of his seat in the Republican column that worked against Tower. Rather, it would be his removal as the new chairman of the Armed Services Committee. i

Next in terms of seniority to head that panel is 71-year-old Barry Goldwater (R-Ariz.).While Goldwater, a general in the Air Force Reserve, is a venerable figure within the party and a staunch advocate of a strong defense, sources say it was thought that his overall knowledge of defense issues, outside of aircraft, is not as great as Tower's, and that the 55-year-old Tower will be more energetic and able to control the committee's Democrats.

Though the Republicans' election landslide wiped out several Democratic legislators who in the past frequently challenged Pentagon programs, a few remain, and some -- including Colorado's Gary Hart and Michigan's Carl Levin -- are on the Armed Services panel.

Goldwater, who stands to become chairman of the Senate Intelligence Committee, has not said whether he would take the Armed Services job instead if it were available. But apparently enough people thought he would do so to make it less attractive for Reagan to select Tower.

Tower reportedly had strong support from some key people, including businessmen Holmes Tuttle and Justin Dart, members of Reagan's "kitchen cabinet" of close advisers. He also had support from Sen. Paul Laxalt (R-Nev.), a close friend of Reagan, and Clements. But the latter's support was said to have faded recently.

It was reported earlier, based on comments by some transition team members that Clements might appoint John B. Connally, secretary of the treasury in the Nixon administration, to Tower's Senate seat and that Connally would have a good chance of winning the special election.

Connally, however, apparently does not want the Senate job. He is known to have wanted the Pentagon post all along. Sources say that his name is now resurfacing, along with those of Donald Rumsfeld, secretary of defense in the Ford administration, and Sen. Henry M. Jackson (D-Wash.).