That's because the Enemy had the Democrats on their side, without the Democrats they could not have won. Make Richard Nixon a Dictator and he could have crushed North Vietnam.
I don't care what your politics are, beliefs like this are dangerous and scary.
Actually, making Nixon a dictator probably wouldn't have changed anything because it was Nixon who went for a Vietnamization time-table that was too rapid. His plan all along was withdrawl and stalemate, not victory.
There is an argument that Democrats caused the fall of South Vietnam by calling in all our loans, cutting off all their arms and spare parts, and then putting an arms embargo on them. From that perspective, we actually won the Vietnam War because we switched sides.
However, Nixon was the one who pushed for us to get out of the war through "Vietnamization", several years before the South was remotely trained and equipped enough to handle the task. MACV wanted to stay for two or three more years, Nixon made them leave on a time-table they didn't think was acceptable. However, he did so under domestic pressure, not thinking there were other options because the Pentagon failed to see all the better options (and there were many that would've resulted in far lower troop numbers, far less combat, no bombing, and no Northern victory).
Ford was the one who threw out our treaty commitment to come to the South's aid if the North rolled south. Based on the 1973 action, our airpower would've obliterated the North's armored forces moving through the central highlands in '75, requring the North to spend 2 to 5 more years rebuilding their army, by which time the South should've become too economically dominant for the North to be a realistic rival (like South Korea vs. North Korea, or West Germany vs. East Germany). If the South had made it to the 1977 to 1980 time frame, the North probably would've been forced to give up any idea of invasion. The 1973 fighting had left them scared and demoralized because the South Vietnamese troops were already outfighting them, man-for-man. In '75 the South's soldiers were out of ammunition before the battle even started, so it didn't matter how good they were.
In any event, the US military and our advisors were also the ones who left Vietnamese forces positioned to fight an insurgency instead of a conventional invasion, and who never had the clear insight that the war was actually the same as Korea, where half a country wanted to invade the other half (the North Vietnamese depended on us misperceiving the war as a revolution, instead of a conventional large-army civil war between two distinct regions). Who failed to build a defensible DMZ, and failed to provide for a credible seaborne invasion threat to make the North think twice about sending their entire army south, leaving the North itself unprotected (the Inchon scenario).
Our military and political advisors were also the ones who thought the main combatants were the South Vietnamese Army and US versus the Viet Cong and NVA, leaving out the largest combatant force, the South Vietnamese popular forces, who were given little bits of junk here and there for most of the war. We armed them far too late, with far too little, even though they were actually doing the bulk of the defensive fighting.
MACV largely corrected many of the earlier mistakes with a change in policy in 1969 or so, when Creighton Abrahms replaced Westmoreland, but under Nixon's timetable they just didn't have time to get the South fully ready to go it alone.
Interestingly, before the anti-war protests started, it was conservatives who were questioning the war, since what we were doing over there didn't seem to make a lick of strategic or tactical sense, like we were just mucking around.