Man......this one is tough. Without Chappaquiddick and if Tricky Dicky is up to his dirty tricks it could either mean Tricky Dicky still manages to score a landslide election (if he doesn't get caught), or it could mean that Ted takes full advantage of Watergate (when it first comes out to public knowledge) and score a decisive victory against Nixon. I imagine the latter is more likely to happen, as IRL Ted was pretty bright about the whole situation.
It is an unfair assumption that Edward Kennedy would be unable to connect with Southerners and Westerners, actually due to his being a Kennedy and thus American Royalty would actually help him with traditionally Democratic places especially if he picks a Southerner like Terry Sanford, Lloyd Bensen, or even Fred Harris (though that might be the Okie hack in me speaking). I believe that if he ever made it to a general election and didn't have Chappaquiddick on his record Ted Kennedy, no matter how liberal he was or appeared to be, would be able to work with the American people to get him elected like he did with Congressmembers across the aisle to pass the legislation he really cared about. Just saying. And like I said earlier Ted would milk Watergate for all it's worth.
Here is how I think an Edward Kennedy win against Nixon best case scenario, all guns blazing:
Richard Nixon (NY)/Spiro Agnew (MD) (Republican) 100 evsEdward Kennedy (MA)/Lloyd Bentsen (TX) (Democratic) 438 evsThis sounds insane, but I believe this is possible.
A moderate Kennedy win:
Richard Nixon (NY)/Spiro Agnew (MD) (Republican) 205 evsEdward Kennedy (MA)/Birch Bayh (IN) (Democratic) 333 evsA lean Kennedy win (most likely):
Richard Nixon (NY)/Spiro Agnew (MD) (Republican) 242 evsEdward Kennedy (MA)/ Mo Udall (AZ) (Democratic) 296 evsSounds crazy at first, but think about the family name, how effective of a Senator Ted was, Watergate, and possibly even Spiro Agnew's controversy. It's easy to hide this stuff when your opponent is George McGovern, sure as hell won't be easy when it is Edward M. Kennedy.