Well, don't forget that inflation was high and the economy was poor in 1976. Plus, in all but one post-1952 election, the White House has switched parties after eight years.
I'm not so sure the nominee would be Scoop, who always had some support among the DC establishment but who rank-and-file Democrats never particularly liked. In real-life he fell completely flat not because of Watergate but because he unwisely skipped Iowa and NH and because liberals in the party hated him but thought they could at least stomach Carter. He might do okay if he sweeps the Southern primaries, but he'd face competition from George Wallace and quite possibly Jimmy Carter.
I know this goes well against CW, but I actually don't know that Carter can be completely counted out. Yes, the CW is that he was purely a response to Watergate, but there were a number of other factors at play that wouldn't go away in this TL. Remember that Carter was the only Democrat to actually understand the new primary rules, contesting every state including all the early ones. There was also deep dissatisfaction among Democrats with the party's DC establishment - dissatisfaction that had little do with Watergate and a whole lot do with issues like Vietnam and the battles between Labour and the New Left. And Carter was also the only candidate who was able to get the support of Southern Dems while holding onto liberals. So I think he'd be a factor in the Democratic primaries even absent Watergate.
In short, though, I'd say the Democratic field is extremely difficult to predict. It might have gone to the convention. Jerry Brown, Jimmy Carter, Mo Udall, or a compromise ticket headed by some Labor-friendly generic Dem like Walter Mondale or Adlai Stevenson III. Humphrey wouldn't have run because he knew he was dying of cancer. And Ted Kennedy would sit out because of Chappaquiddick and his own ambivalence about being president.
I think RB's right that Reagan would be the probable Republican nominee, but his odds would be no better than 50/50, especially considering how extreme he was viewed by most of the public at the time. I suppose one interesting question is whether Reagan could then come back in 1980 for a repeat nomination and win assuming OTL conditions between 1977 and 1980.