Agnew still resigns without Watergate, so Nixon still replaces him and likely does so with Ford as Connolly would never get through congress due to Democrats seeing him as a traitor and Republicans not trusting him. Plus, IIRC, Connolly had baggage of his own. If not Ford, Nixon goes with Rocky, HW Bush, or even despite how he felt about him personally, Reagan. Regardless of what Nixon wants, Reagan still runs for President with Rockefeller, Charles Percy, or if he can be convinced, Ford serving as the moderate voice in the primaries. Given how close the Republican primaries were in OTL and due to Nixon not being able to push his dream candidate Connolly through, Reagan is likely nominated. He picks Schweicker or Percy to be VP to appease the moderates.
On the Democratic side, Carter still runs but gets no traction without Watergate. Scoop Jackson, Frank Church, Mo Udall, and all the other '76 candidates still run. If the field remains unchanged from OTL, Jackson or Udall get nominated for the Democrats with a moderate southerner or someone from the Rust Belt getting picked as VP.
The General election campaign depends on a variety of factors. Does Nixon go all out for Reagan when he secures the nomination? Or does he do to Reagan what Ike did to him in 60 in the hopes that someone more Nixonian gets the nomination in 1980 after a Reagan loss? If it's Jackson that's nominated, does the anti war left sit the election out? Or do they hold their noses and vote for him due to domestic issues they might agree with him on or just for the sake of stopping Reagan? Electorally, the GOP has the advantage as the Democratic grip on the south at the Presidential level was gone, and the GOP still had sway in the North East and the West Coast. The Democrats on the other hand have a sour economy and inflation (which will be just as bad or worse with Nixon still being there than it was with Ford OTL) working in their favor. With Jackson, it's a toss up IMHO, with Udall, so long as he picks a running mate from the south or rust belt has a slight advantage. Whoever wins, especially if it's Reagan or another Republican, is favored to lose in 1980 though.