Sudan's leader faces toughest threat yet

The obvious is now official: Sudan's pro-US leader is facing perhaps the toughest threat yet to his 15-year rule. By announcing martial law Sunday and issuing a series of decrees since then, President Jaafar Nimeiry has in effect acknowledged the failure of bids to quell a rebellion in Sudan's south and defuse growing social and political tension in the north.

His series of sudden political moves, including the announcement Wednesday of a major Cabinet shuffle, also amounts to recognition that the trouble is much more serious than he has suggested.

Some Mideast analysts also view the martial-law declaration as a signal Mr. Nimeiry felt that support from traditional allies - neighboring Egypt and the United States - could not alone ensure victory over growing opposition to his rule. The US and Egypt, diplomats say, have made clear to the Sudanese leader their feeling that any fundamental solution to his problems must start with the recognition that they are not only the work of external enemies, such as neighboring Libya.

The Egyptians' misgivings became especially evident following an air strike in March on the outskirts of the Sudanese capital - an attack the Sudanese, Egyptians, and Americans all blamed on Libya. The Egyptians offered strong verbal support for Nimeiry, and made it clear they were committed to defeating any Libyan move to topple him. But the Egyptian defense minister also stressed publicly that Cairo maintained only a tiny troop contingent next door, and that Egyptian policy was to steer clear of ''the internal affairs of Sudan.''

Sudan was the only Arab country to stand fully behind peace moves by Egypt's late President, Anwar Sadat, with Israel.

The uprising in the Sudanese south, an impoverished non-Muslim region in a majority-Islamic state, has indeed received open backing from Libya. The unrest gathered steam when Nimeiry declared late last year that Islamic law would henceforth be the law of the land.

The southern rebels are also incensed at what they see as a retreat by Nimeiry on guarantees of political autonomy for the area. These assurances were codified in a 1972 accord that lumped the south's three constituent provinces into a single, largely autonomous, region and ended nearly two decades of south-north unrest.

Amid the renewed southern unrest, opposition to the regime has intensified in the capital, Khartoum, and elsewhere in the north. A series of strikes has defied presidential authority. Recent reports speak also of deepening opposition to the regime among university students.

Most worrisome for the regime, presumably, is the specter of united opposition from south and north. In years past, southern political activity has centered on demands for autonomy or, at most, secession. But in recent months, a new umbrella opposition force in the south has appealed for ''a progressive and socialist movement aimed at the liberation of northern as well as southern Sudan.''

In declaring martial law, Nimeiry slightly eased his longtime contention that outside powers, especially Libya, were the sole culprits. He spoke of the need to counter unrest ''both inside the country and abroad.''

The announcement does not necessarily rule out further wielding of the carrot as well as the stick. In recent weeks, Nimeiry has moved to meet some of the opposition's economic gripes, and has offered amnesty to all southern rebels who lay down their arms. But there has been no sign so far he is willing to retreat on the issue of Islamic law.

The main emphasis in recent days has been on the stick. After martial law, Nimeiry issued decrees banning demonstrations, further constraining an already pliant press, and widening the authorities' powers of search and curfew.

During his rule, Nimeiry has survived almost as many coup attempts as Pauline has evaded Hollywood perils.

But the concern among diplomats now is that he faces less easily defused opposition.