Analysts of North Yemen's guerrilla war agree that the National Democratic Front does not yet pose a serious military threat to Sanaa, whose forces are far superior. The main danger, most seem to believe, is a political one.

According to diplomatic sources and press reports, the front consists of 700 to 800 hard-core Marxists, an estimated 500 defectors from the North Yemen Army and somewhere between 200 and 6,000 guerrillas. Many of the latter are said to be simple tribesmen fighting to redress local grievances rather than for ideological reasons.

Founded in February 1976, the front long was regarded as a little more than a pawn of the Marxist South Yemeni government in its struggle to unite the two Yemens under its control. But with growing outside support from Syria and Libya, among others, the front seems to have taken on a life of its own.

Front guerrillas were involved in the invasion of the north by regular South Yemeni units in February 1979. That crisis triggered alarm bells in Riyadh and Washington and led to the rush of American arms to Sanaa as well as the successful Arab League mediation that ended the brief war.

Since then, the two Yemens have tried to outmaneuver each other in perennial unification talks. But despite a formal agreement on a constitution for a united Yemen in early January, the guerrilla fighting escalated recently with the shooting down of two airplanes.

Nonetheless, some long-time war watchers believe that the government's position has improved considerably since last August, when front guerrillas held a mountain overlooking the town of Ibb in south central Yemen and were in position to cut the main Sanaa-to-Taiz road.

The 30,000-strong Army took the initiative, drove the guerrillas off the mountain and relieved all pressure on both the town and road.

In January 1980, the front signed a cease-fire accord with President Saleh and agreed to a role in the government.

But the agreement, like so many others signed over the years between the North and South, was never carried out. Saleh decided instead on a military campaign against the front to wipe it out.

This tactic, however, appears to have backfired politically and won Saleh no sympathy with those in a position of influence with the front.

Arab press reports, accepted as fairly accurate by diplomatic sources here, say that Saleh failed during his trip to the Soviet Union last October, and in subsequent talks with Syrian and Libyan leaders on the way home, to gain a commitment from any of them to pressure the front to ease its military activities.