I'd really dispute the claim that Obama is more to the left, economically, than Clinton, let alone Carter. Taxes are lower than they were under Clinton, and it's Obama's policy that they should stay that way. Obama care is more market-oriented than Hillarycare was, and is derived fairly explicitly from the kind of ideas that the Heritage Foundation liked in the '90s, as a way of forestalling a genuinely leftist set of reforms.
Bill Clinton and Jimmy Carter both left economic deregulation as their chief economic accomplishments. Clinton deregulated the financial industry; Carter deregulated airlines and trucking. Obama has enacted more new federal regulations than any president since Nixon.
Obama wants higher taxes on the wealthy; Clinton has publicly disagreed.
And Carter did not support any attempt at universal healthcare as president. He did not believe people had a "right" to healthcare. Nixon was more of a liberal on healthcare than Carter.