Two years after the light aircraft crash that killed then-prime minister Francisco Sa Carneiro, a group of his followers opposed to the leadership of Sa Carneiro's successor have begun a campaign to open a fifth inquiry into the cause of the crash.

The campaign is a reflection of partisan battles within the governing Democratic Alliance, the center-right coalition Sa Carneiro headed, and the tenuous leadership of Prime Minister Francisco Pinto Balsemao on the eve of hotly contested local elections here.

Sa Carneiro died in December 1980 when the twin-engine aircraft plunged into a residential suburb after takeoff from Lisbon airport. The other six victims included his Swedish wife and his defense minister.

Sa Carneiro's death nearly resulted in the breakup of the alliance, composed of the fractious Social Democratic Party he headed and its right-wing junior partner, the Christian Democrats. The choice of Balsemao as leader of the Alliance was a compromise and has since been a constant source of friction within each of the parties and the coalition itself.

There have been four investigations of the crash, two of which have not yet officially concluded. All seem to rule out any hint of crime, but allegations have persisted of politically motivated sabotage.

The current campaign, launched by 51 deputies petitioning for a new parliamentary inquiry based on what they say is fresh evidence, is viewed here as an attempt to discredit Balsemao by implicitly charging him with a political cover-up.

The deputies, dissident hard-liners within the coalition and stern disciples of Sa Carneiro, have the support of opposition Socialist Party leader and former prime minister Mario Soares, who has reversed his previous publicly expressed belief that the crash was an accident.

The deputies say two files containing highly sensitive documents relating to state security disappeared from the wreckage immediately after the crash. The conservative press also has alleged that fragments of metal found on the exhumed bodies of the two pilots point to an explosion.

The charges come barely two weeks before nationwide local elections that will be the first public judgment on Balsemao's leadership since he took office.

Attorney General Augusto Chaves, lamenting what he called "the cyclical reappearance of sabotage charges every time there is a political crisis or election campaign, last week revealed some of the conclusions of the ongoing inquiries, including an exhaustive report by a team of nine U.S. specialists from the National Transport Safety Board confirming earlier conclusions that there were no grounds to suspect sabotage.

Their report points to pilot error, establishing, among several other contributing factors, that the aircraft's left engine failed minutes after takeoff and that the pilot failed to "feather" the propeller blades--the only way to pull out of that type of emergency.