FOR three years, North Vietnam has dumb-founded U.S. strategists by withstanding the heaviest aerial onslaught in history. Faced with an unshaken enemy, the Pentagon no longer contends that it can bomb Hanoi to the negotiating table, insisting instead that the North is "paving" dearly for its continued support of the southern insurgency. Even this revised assessment is not as safe as it seems. Though somewhat patchy, the evidence from Hanoi indicates that U.S. bombing is making the government and social structure of North Vietnam stronger than ever.

Reconaissance photos and statistics miss the essence of the impact of U.S. bombing on the North. In dispatches from Hanoi and subsequent Senate testimony. Harrison Salisbury repeatedly argued that the damage caused by bombing was more than balanced by the popular patriotism and self-sacrifice it aroused. The new popular unity and mobilization seems only part of the picture.

To begin with, North Vietnam's Community Party (DLD) will probably emerge from the holocaust with unparalleled authority and support. As the political structure presently organizing mass resistence activity it stands to gain most from popular anger with the U.S. The objective of fighting the aggressor seems likely to mesh inextricably with the party's political goals.

COMMUNISTS often try to identify their program with nationalism, but that identification is usually most complete during revolutions, before the party has unveiled its programs and alienated sizable groups of the populace. The DLD is known to be having trouble with its intelligentsia, which is still smarting from the clamp-down which followed a brief period of academic liberalization in 1955. This new period of truncated revolution that U.S. bombing has brought about is giving the DLD a rare chance to bind old wounds.

The bombing has not prevented the DLD from doing some long-range planning and construction. While crisis solidifies its war-time authority, the DLD has been quietly laying the foundations for North Vietnam's post-war society. Surprisingly, its blueprint does not involve a tightening of central reigns. In the past three years, Ho Chi Minh has steadily decentralized almost every aspect of North Vietnamese life, scattering industry and schools throughout the caves, jungles, and villages of the North's back-country. Limited decision-making power is sifting down to local cadre.

UNDOUBTEDLY, this strategy is partly an interim response to American bombing. Decentralization protects people and machines, and encourages local self-sufficiency facilitating the problem of distributing goods in the face of air harrassment.

But important features of the new program may survive the war. The regime's new policy of encouraging local initiative is firmly in line with Mao Tse-tung's ideas on winning mass support and utilizing it most effectively. While not an ideologue by any means, Ho Chi Minh is well aware of the value of many of the Chairman's techniques (the DLD party statutes specifically encourage the study of the "thoughts of Mao Tse-tung"), and this seems to be one idea he's adopted as a long-range objective.

The main advantage of the tactic is that it combats over-centralization by permitting low-level cadre to adopt broad party policy to local problems Premier Pham Van Dong and President Ho are well aware of the dangers of applying party principles inflexibly. In interviews with visiting European communists last summer, Pham emphasized that the regime was taking measures to guard against creeping bureaucratization. He and Ho have had some near disastrous experiences dictating indiscriminately from the top. Hanoi's rigid land reform program of 1955-56 produced a revolt in Ho's home province in November, 1956.

Permitting some local independence will not threaten the ultimate power of the central authorities. By giving low-level cadre and involved peasants a sense of participation, decentralization identifies them more closely with the regime. Among a populace traditionally powerless, a little authority goes a long way in this direction.

HANOI'S war-time strategy could have important effects in what the communists call the "cultural" sphere as well. By dispersing city populations throughout rural areas, it is bringing more politically conscious urban workers and university students into contact with the North's relatively backward peasant masses. While ostensibly the product of a mass revolution. North Vietnam seems to be having real problems with its peasantry, which has resisted regimentation more persistently than China's rural classes. The regime may be hoping the example of a dedicated urban phalanx and the patient persuasion of students will bring around some recalcitrant farmers.

Hanoi undoubtedly sees the dispersal and hardship of war as a lesson for its urban elite as well. The DLD echoed Mao's concern with getting intellectuals to develop a feel for the peasantry when it packed students off to the hinterland for several months of manual labor of 1958. Now students scattered in jungle and mountain retreats are hacking out their own classrooms, sleeping in straw huts, and walking dirt paths to classes.

Decentralization, then, aims also at promoting an even cultural mix--creating a truly classless society. On this score, it seems to have gotten mixed results. In May, Pham Van Dong spoke earnestly with an Italian physicist about the "cultural" benefits which the war was bringing peasants, but his insistence seemed to hid frustration, and the mere mention of the problem establishes its existence.

IF A cultural gap persists, it is not surprising. North Vietnam has been historically an elitist society, dominated by mandarins even more estranged from the peasantry than their better-known Chinese counterparts. And Hanoi, while destroying traditional class lines, has helped replace the old elite with another by consistently favoring its heavy industry and relying on its urban classes to lead the country out of poverty.

Still, decentralization--combined with the unifying effect of the bombing threat--represents a formidable assault against class lines. And the longer the bombing persists the better are Hanoi's chances of welding its people together.

There is no denying, of course, that U.S. bombing has hurt Hanoi's economy badly. Its new industrial facilities are rubble, and all-important agricultural labor has been diverted to repair communication lines crippled by the U.S. bombing.

But bombing is not going to cripple Hanoi permanently, any more than it has hobbled its present war-effort. Communist bloc aid will spur Hanoi's industrial recovery. The regime demonstrated its ability to raise itself by the bootstraps after it took control of the war-ravished North in 1954.

The ultimate irony of U.S. bombing is that Hanoi may look back on its years under fire not as a period of national disaster but as a golden age of Vietnamese Communism.