Madrid — Rumors of the possibility of another coup here have reached a crescendo with the threatened disintegration of the ruling Democratic Center Union party (UCD). Fifteen social democrats in parliament abandoned the UCD Nov. 4, creating the biggest internal crisis since Premier Adolfo Suarez's resignation, which preceded last February's aborted military coup.
One way for the UCD to pull itself together would be for the present party president to resign and for Leopoldo Calvo Sotelo to take over that job, along with the premiership, which he now holds. This possibility seems most likely if the UCD can opt for a unified ''moderate'' program supported by the Suarez group or a more conservative orientation to form a coalition with right-wing parliamentary groups.
But although the social democrats promise to continue parliamentary support to the government, they could give the opposition Socialist Party a working majority in a left-wing coalition.
Coinciding with the internal crisis of the government party, the official Army bulletin published an astonishing award of a medal to indicted coup-plotter Gen. Jaime Milans del Bosch for ''patriotic suffering'' from a minor wound received over a year ago in a rough helicopter landing.
The Defense Ministry revoked the award late Wednesday night, but concern over the possibility of another coup attempt gainedmomentum.
The award was preceded by a series of military promotions of officers of dubious loyalty to democracy and with known sympathies for the coup-plotters. These created a public impression of leniency and lent strength to the assumption that Mr. Calvo Sotelo had made a deal with the military: freedom to promote whom they wished in exchange for noninterference in civilian matters.
Perhaps to play down this impression, the Supreme Military Council decided Thursday to promote two generals who helped stop the February coup, Gen. Aramburu Topete of the civil guard and Gen. Saez de Santa Maria, chief of the national police.
Money is being collected openly in support of the indicted plotters of February's coup, and even, some Spanish newspapers report, in support of another coup, perhaps near the sixth anniversary of Franco's death, Nov. 20.
Both the government and the opposition Socialists caution that dissolving parliament and holding early elections would only increase political instability.
The social democrats, who defected because of the UCD's increasing right-wing orientation, have guaranteed support of Calvo Sotelo for the sake of stability but will likely form a ''hinge'' party called Democratic Action, swinging left or right according to the issue.
In a reference to the social democrats, the Socialists have indicated that they would accept ''independents'' on their 1983 electoral lists.
Socialist leader Felipe Gonzalez has insisted on the need for calm. He has pointed out that the other variables that could contribute toward a coup - such as rise of terrorism, regional autonomy conflicts, labor unrest, and street crime - are at all-time lows and there should be no need to go into hysterics over a crisis in a democratic party.
As one leading businessman said, ''The Spanish UCD contains the entire German parliament in one party, from social democrats, center moderates, liberals, and Christian Democrats to ultraconservatives. It's only natural for it to split.'