If not for Jimmy Carter who would have been the Democratic nominee in 1976?

How did Carter lose such a lead? I know about the interview he gave to Playboy magazine, but was there something else? Not that anyone would win with such figures. LBJ still has the all-time record for highest popular vote win, and he managed 61.1%.
To be fair, Carter had that lead with the post-Convention bounce. Look at Romney now, his bounce after becoming the nominee has slipped and Obama's leading 48-43 now. Ford then got his convention bounce, and kept chipping away at Carter's lead because he ran a very good campaign. Carter just wasn't exciting.

Gallup said:
The 1976 Election
Given his mediocre approval ratings, Ford faced a difficult bid to be elected president in his own right. Additionally, because he was an "accidental" president -- elected neither as president nor as vice president -- his party's presidential nomination was not assured, as it is for most incumbents.
Republican Nomination Campaign
Reagan, the former California governor, challenged Ford for the 1976 Republican presidential nomination. Ford held a solid lead over Reagan and other prospective challengers throughout most of 1975 in Gallup's nomination preference polling of rank-and-file Republicans. In November 1975, shortly after he officially announced his candidacy, Reagan overtook Ford by 40% to 32%. That proved to be temporary, as Ford regained the lead over the field in early January 1976.
pr060808bii.gif
With only Reagan and Ford as viable candidates, Gallup asked Republicans for their preferences between the two men for the duration of the nomination campaign. Ford led throughout on this measure, though Reagan narrowed the gap on several occasions. Ford led by 20 points or more through most of the spring, before Reagan pulled closer in late May and early June. However, after the primaries were over, Ford's lead again expanded to more than 20 points, and in the final test just a few weeks before the Republican convention, he led Reagan by a 61% to 35% margin. Despite Ford's large lead among rank-and-file Republicans, the nomination was still in doubt during the party's convention, and he won a narrow vote over Reagan among the delegates.
pr060808biii.gif
General Election Campaign
In presidential trial-heat matchups between Ford and Carter, Ford trailed the eventual Democratic nominee by small margins in March, April, and early May. By late May, Carter opened up a double-digit lead and maintained it until late September. Carter's lead swelled to as much as 33 points, 62% to 29% among registered voters, after the Democratic convention that year. Ford cut into the margin after the Republican convention, reducing a 25-point (57% to 32%) early August deficit to 13 points (50% to 37%).
pr060808biv.gif
In a poll conducted immediately after their first debate, Carter maintained a double-digit lead. But Ford chipped away at that lead and before the second debate a poll showed the two in a statistical dead heat (Carter 47%, Ford 45%). However, in the next debate Ford incorrectly asserted that Eastern Europe was not under Soviet domination, and Carter pulled ahead, 48% to 42%. Carter continued to lead the race for the next few weeks, but Ford made up additional ground following the third debate in late October, again pulling even.
In the final pre-election poll, Gallup's numbers indicated a statistical dead heat among likely voters, with Ford at 49% and Carter 48% (the unallocated numbers had Ford at 47% and Carter at 46%). The actual outcome was 50% for Carter and 48% for Ford. The election was so close that it was not certain that Carter would win until the morning after Election Day.
 
Top