The right had been ascendant since 1964, Nixon was merely a bridge between the declining Rockefeller wing and the ascendant movement Conservatives. The right would've taken over eventually, if not in the 1980s, sometime in the 90s or 2000s. My question is, if 1976 goes Republican without Watergate, or a better Democratic politician than Carter gets elected, how do the Democrats fair?
Those are two vastly different scenarios, but the problems in the late 1970s were more structural than anything.
The US Economy's boom times from 1945 to 1972 were largely based off of the fact that the rest of the world was either postcolonial and nonindustrial, a bombed out husk of its former industrial self, or were inefficient Marxist states. The upsurge in Germany and Japan exposed how noncompetitive American industry was, and how much industrial subsidies were propping up sectors of the economy that needed reform. There wasn't enough true growth happening and incumbents were slacking off in R&D expenditure. The New Deal Consensus relied on the notion that the economy was just going to keep growing and that a troika of government, labor, and established industries could largely direct the US economy towards sustainable and equal growth.
The OPEC crisis in 1973 and the global crop failure of 1972 showed this to be hollow. Whoever was in government in the mid to late 1970s was going to have to deal with the fact that while inflation was sort of baked in to most economic models, growth from industrial policy was starting to bring back diminishing returns. If it wasn't for the Vietnam War and the growth in defense spending and business for military adjacent companies (a big sector of the US economy, particularly in the Sunbelt), this would have been clear a lot earlier.
Carter was absolutely horrible at working with Congress and I think that was his biggest issue. Now, if Carter doesn't enter,George Wallace does a lot better and might be able to almost win the South as a bloc in the primaries. He was far more popular than he is given credit for, and in his campaigns in 1972 and 1976, he went far beyond the Bircher constituency. There is no way the Democratic Party lets Wallace win, but in this case, I think Scoop Jackson makes some inroads and wins because Mo Udall had issues that were not resolvable with labor and other big Democratic constituencies.
I think a President Scoop Jackson alienates the left big time because unlike Carter, he wouldn't abandon American allies like the Shah of Iran, or President Somoza in Nicaragua, nor do I think he would give up the Panama Canal. He might be a bit more amenable to SALT, though. I think his Presidency would in many ways be defined by foreign policy; keep in mind that Carter was operating from a position of weakness, which helped him to shepherd along his ideas on foreign policy; Jackson would fight against that.
He however would be an explicitly labor focused President and would do things that they wanted, which might make the economic crisis developing worsen with government bailouts and the like.