By just about any measure the Democrat have not fielded a real liberal in many decades. Obama is a moderate, he is more right than Bush in some areas.
The real question is why are Republicans so anti-reality, anti-science and anti-education? Do they really want the country led purely by personality and no brains? Bunch of stiff white pretty guys and easy on the eyes no-nothing women. Give me an ugly, smart woman or short fat smart guy, it is all style and no substance on the right.
The last Democrat liberal who won was JFK, who was a centrist-liberal, and before that FDR, who was a man from the wealthy class who happened to be President during an extraordinary period of the country turning toward socialism, which he wanted to prevent, and a major crash. Carter was more a moderate too.
Clinton's main liberal claim was a tiny tax increase on the top 2% - when he wasn't deregulating Wall Street, slashing Welfare and other social programs, etc.
As far as candidates, Kerry was a moderate Democrat, as was Gore (saving the planet isn't 'left', the fact the right opposes it is just that they're ignorant sellouts). Gore's VP nominee was Joe Liebermann of all people who promptly endorsed the Republican the following election (that's happened with the Republicans when?)
George McGovern, who lost 60 of 50 states, was more a 'liberal'. LBJ was a liberal domestically (equal rights, how radical leftist), but a cold warrior.
The Democratic Party just isn't that liberal with its presidents, despite the Republican calling every nominee for several elections 'The most left-wing member of Congress!'