Obama is a Democrat, and the Democratic Party courts liberals as major constituency but,

parties change over time. Until WWI, the Democratic Party was not liberal. By the time of Truman, when Blacks were allowed in the Party, we had moved to a center left, with a strong racist southern constituency. Northeaster Republicans were more socially liberal than many in the Democratic Party. When Johnson passed the Civil Rights Act of 1964, he lost the South. Nixon's Southern strategy brought Southern Democrats into the Republican fold and Reagan completed that process, turning Reagan Democrats into a Republican South. With Reagan, Northeastern socially liberal Republicans began to move to the Democratic Party. This process ended with Obama as the North East is heavily Democratic and socially liberal.

The entire country shifted right from Nixon through Bush. Even Jimmy Carter was a Social liberal fiscal Democrat a bit to the right of Johnson. Clinton was to the right of Carter. Oh, he tried a few liberal things higher taxes, and bringing gays more into the mainstream. (At the time, don't ask don't tell was considered an abomination in the sight of God. I lived through it, in the military. It lasted too long.) But Clinton's fiscal policies and deregulation were comfortably Reaganist.

2006 began the move back toward the left in reaction to massive Bush and Republican mismanagement. Obama is not a liberal in classic political terms, or even compared to Johnson or Kennedy. But he has moved us back a bit to the left in social issues. The Affordable Care Act, (a thoroughly Republican plan circa 1990), revealing that the Republicans had moved so far right that they could not even agree with solid Republican Policy of the 90's. He was slow to accept gay marriage, but finally ended Don't Ask Don't Tell and does not defend DOMA. He has appointed center left Justices. He continues the Imperial policies of Bush (foreign wars,war on terror, interventionist, spying on American Citizens) and has claimed the right to kill American Citizens without due process.

He sits, I think, at the center of American Politics, which would be center right under Bush, more than Center Right under Clinton, Right under Reagan, etc. etc. The momentum of the American public is left now. President Obama can slow that down by fighting it or help it along by going with the public.

He is still limited by what will pass the most right wing Republican Party in history. This isn't going to change until, at least, January of 2015, and only if we elect a Democratic Congress.

He was the only choice. Even if Hillary Clinton won the nomination, there would have been little difference. Republicans would have been as intransigent under her as they have been under Obama. No more liberal candidate was in the running.