In terms of pure numbers across the separation of powers (the GOP will maintain the minority's filibuster power in the Senate), I don't think Barack Obama's electoral victory constitutes the sweeping political earthquake that the concept of a critical realignment implies (see Jay Cost's detailed examination of the question: "Is 2008 a Realignment?").
That said, the more I read and reflect on the genuine change coming to America in an Obama administration, it's clear we've no doubt witnessed what might be termed a "qualitative realignment," meaning that the philosophical sensibilities of the electorate have be so transformed that the election signifies an undeniable and lasting break with the past.
George Packer, at the New Yorker, captures the essence of this qualitative realignment:
Barack Obama’s decisive defeat of John McCain is the most important victory of a Democratic candidate since 1932. It brings to a close another conservative era, one that rose amid the ashes of the New Deal coalition in the late sixties, consolidated its power with the election of Ronald Reagan, in 1980, and immolated itself during the Presidency of George W. Bush. Obama will enter the White House at a moment of economic crisis worse than anything the nation has seen since the Great Depression; the old assumptions of free-market fundamentalism have, like a charlatan’s incantations, failed to work, and the need for some “new machinery” is painfully obvious. But what philosophy of government will characterize it?That's probably about as well as it can be said, without sounding full of hubris and spite. As always, we'll really know if 2008 was a realignment in the years ahead, because the theory's essentially "retrodictive" in its explanatory foundations.
The answer was given three days before the election by a soldier and memoirist of the Reagan revolution, Peggy Noonan, who wrote in the Wall Street Journal, “Something new is happening in America. It is the imminent arrival of a new liberal moment.” The Journal’s editorial page anticipated with dread “one of the most profound political and ideological shifts in U.S. history. Liberals would dominate the entire government in a way they haven’t since 1965, or 1933. In other words, the election would mark the restoration of the activist government that fell out of public favor in the 1970s.” The Journal’s nightmare scenario of America under President Obama and a Democratic Congress included health care for all, a green revolution, expanded voting rights, due process for terror suspects, more powerful unions, financial regulation, and a shift of the tax burden upward. (If the editorial had had more space, full employment and the conquest of disease might have made the list.)
For the first time since the Johnson Administration, the idea that government should take bold action to create equal opportunity for all citizens doesn’t have to explain itself in a defensive mumble. That idea is ascendant in 2008 because it answers the times. These political circumstances, even more than the election of the first black American to the highest office, make Obama’s victory historic. Whether his Presidency will be transformative, in the manner of Roosevelt and the handful of predecessors named by F.D.R. in 1932, will depend, in part, on history—it’s unclear whether today’s financial troubles will offer a political challenge, and an opportunity, of the magnitude of the Great Depression. But the power of Obama’s Presidency will ultimately hinge on how he chooses to interpret the “modern application” of liberalism in the twenty-first century.