Republican Sen. John G. Tower of Texas, an outspoken supporter of stronger defenses and higher defense spending, is now a front-runner for the job of secretary of defense in the Reagan adminstration, according to sources within the Reagan transition team. r

If Tower is, in fact, picked for the top Pentagon post by President-elect Ronald Reagan, then sources say it could be part of a Texas-style double-play in which the Republican governor, William P. Clements, then appoints Republican John B. Connally to fill Tower's seat in the U.S. Senate for an interim period until a special election could be held in Texas. Sources say Connally -- a former Texas governor who was also treasury secretary in the Nixon administration -- would likely do very well in such an election.

Reagan met with Connally for two hours on Saturday at the president-elect's California ranch and then met with Clements for a couple of hours on Sunday.

Officials on the transition staff have said that no final decisions on Cabinet appointments have been made yet, and that Reagan's senior advisers are going over the now shortened lists of top candidates. Reagan has announced that he will name his Cabinet by early December at the latest.

Sources close to the situation involving the key Pentagon appointment, however, say they believe a tentative decision has been made in the case of Tower, and syndicated columnists Rowland Evans and Robert Novak are reporting today that Reagan has tapped Tower for the defense job in a decision that Reagan insiders, as of yesterday, felt would not be overturned.

Tower declined to comment on these reports yesterday, saying only that "I'll cross that bridge when I come to it."

These sources also say that aside from Connally, another leading Texas Republican, Anne Armstrong, the former U.S. ambassador to Britain, could be chosen for Tower's Senate seat.

Tower, according to sources close to him, has been interested in the top defense post for some time even though the landslide Reagan election victory that will give control of the Senate to Republicans means that he will become chairman of the powerful Senate Armed Services Committee if he stays on Capitol Hill.

Aside from Tower, the persons known to be under consideration for the Pentagon include Sen. Henry M. Jackson (D-Wash.); Connally; former secretary of defense Donald Rumsfeld, and a former secretary of the Navy, J. William Middendorf.

Jackson, the long-time Democratic senator from Washington with solid credentials as a supporter of strengthened defenses, was considered by many transition insiders to be perhaps the leading candidate. However, several sources said yesterday that the prospect of Jackson being named seemed to be fading fast because of opposition by more conservative and traditional Republicans who feel that the new administration needs a "totally reliable team" rather than one with a leading Democrat in a key role.

Reagan has said that he wanted to have a bipartisan government, however, and other aides say that Reagan retains great confidence in Jackson so that his appointment could not be excluded at this point.

Jackson was also being mentioned as a possible secretary of state, but the same Republican Party forces are said to be working against him there. Among the leading candidates for the secretary of state job mentioned yesterday by senior Reagan aides were retired general Alexander M. Haig, former treasury secretary George P. Shultz and former Nixon budget office director Caspar W. Weinberger. In the unofficial sweepstakes surrounding that key post, transition sources yesterday said they thought Haig's stock was on the rise.

The election results gave the Republicans a 53-to-47 edge in seats in the new Senate that will convene in January. If Tower does move to the Pentagon, Clements could appoint a temporary successor for the Senate job. Texas law requires, however, that a special election be held within 90 days to fill the seat. While a member of sources said that Republican power in Texas was now sufficient to make a victory by Connally, Armstrong or another candidate highly likely, there was at least some risk of losing a seat by shifting Tower and this was said to still be under evaluation in top Republican circles.

Tower came to the Senate in 1961 in a special election after Lyndon B. Johnson became vice president under John F. Kennedy.