Today, with American air power so unchallenged, it seems strange that the Democrats didn't want to allow air support of the South Vietnamese. After all, a couple of decades later, a Democrat President got involved in an internal dispute of negligible significance to America, and bombed Yugoslavia into ceding control of its internationally-recognized Kosovo province, at minimal cost in lost aircraft. The number of planes lost to enemy fire in both Iraq wars has also been tiny.
But, the American advantage in air war was much less overwhelming in the 1970s. We lost 3,322 fixed-wing aircraft in Vietnam, perhaps the majority of that number to enemy fire. During Operation Linebacker I in the middle of 1972 that helped defeat the North Vietnamese armored invasion, 104 US planes were lost in combat. Airmen who survived being shot down often became prisoners (in effect, hostages). Years of negotiations had been required to get the POWs back in early 1973, so there would have been reluctance to follow a policy that would have created new POWs.
So, providing air support was nowhere near as painless as it seems now.
There then followed a half decade of Soviet advances around the world, in contrast to the stability of the strategic balance that had endured during most of the Vietnam War. I'm a big believer in the truism that people love a winner and hate a loser, and losing in Vietnam made the U.S. look like a ... loser.
Fortunately, the Soviets wasted their treasure on useless new Third World allies like Ethiopia, while letting a puppet state that really mattered -- Poland -- go to hell.
Luckily, the Communist dictatorship of united Vietnam proved relatively non-insane, and Vietnam today is a pro-capitalist dictatorship, which the U.S. would have found perfectly acceptable back then. But the Hanoi regime was dedicated to communism and wouldn't give it up no matter how many Tennessee Valley Authority-style dams LBJ offered to build for them. So, war seems to have been completely pointless on both sides. Of course, from the point of view of the Communist Party members, while they may not be communist anymore, their party still hold a complete monopoly on political power. (Interestingly, the Vietnamese dictatorship, unlike, say, the Chinese or Burmese dictatorships, is almost never criticized abroad these days.)
On the other hand, the batting average of East Asian communist countries at being non-insane is quite bad. If Vietnam, Laos, and (possibly) Mongolia behaved less horribly than expected, Cambodia, China, and North Korea were even crazier than imagined. The most obvious analogy for American leaders in the 1960s for Vietnam was Korea, where we had lost 33,000 men to keep the northern communist regime from overrunning the southern capitalist regime.
Whether the Korean War was worth the cost is seldom discussed these days. (Of course, almost nobody ever talks about the Korean War at all.) Today, 57 years after the start of the Korean war, young South Koreans are about half a foot taller than their cousins in North Korea. Ironically, fighting to a draw may have proved the best outcome for the U.S. in the Cold War, because the emerging economic chasm between the two Koreas provided a salutary lesson in the superiority of capitalism over communism.
Why the Vietnamese War proceeded so differently from Korean War is another topic of little interest today. I suspect one important difference was that Korea was a peninsula, so that after the war, the U.S. could adequately defend against another North Korean invasion across the border, which is only 238 km long. In contrast, South Vietnam had long borders with the hapless neutral countries of Laos and Cambodia, which North Vietnam abused as staging grounds.
Second, Korea had been colonized by the Japanese, whom America defeated, not by white men. In Vietnam, America's racial and political ties to the French former colonial masters were detrimental.
Third, North Korea relied upon regular army units for its invasion, which the U.S defeated at Inchon and North Vietnam had to be bailed out by a million Chinese communist regulars. In contrast, the Hanoi regime artfully mixed irregular and regular forces. When the Viet Cong went on the offensive in 1968, they were largely wiped, but the use of irregular tactics normally annoyed and frustrated the Americans. After most American troops were withdrawn, Hanoi shifted to regular warfare (including tanks) for its 1972 and 1974-75 offensives. Presumably, Hanoi learned from communist mistakes in the Korean War.
This Cold War history, however, has mostly academic relevance to today's struggle with Islamic extremists. The Soviet Union was a vast country with a vast military, and erratic but sometimes impressive technological capabilities. Its communist ideology could win converts among the elites of foreign countries on its own.
In contrast, Islam has virtually no appeal to anyone above the lowest orders of society if they weren't born into a Muslim family. There is no single Islamic superpower to provide direction to the squabbling Muslim states, and most of these governments are more or less averse to the extremists. Even taken together, all the Muslim states in the world have only a small fraction of America's military might. For example, there is no Muslim aircraft carrier. Technologically, Pakistan is 50 years behind America in the development of nuclear weapons, and the rest lag even farther.