The real realignment occurred during the Reagan/Bush years. Before 1980 some states (Texas, South Carolina, Mississippi, and Alabama) usually voted for Democratic nominees for President and some states (PA, MI, IL, CA) usually voted for Republican nominees; beginning in 1992 that was inverted. The once-powerful liberal wing of the GOP began to die off in the North and west as the Republicans took over in the South. Right-wingers rode the "Reagan Revolution" in the North and West until liberals caught on -- and started voting for Democrats. In the South, white Democrats steadily abandoned the Democratic Party in favor of the GOP. A political party staring at the prospect of becoming an impotent party of a permanent minority reorganizes itself, and tries to find and exploit ideological splits between conflicting interests within the dominant Party. The Democrats did exactly that between 1980 and 1992... and succeeded; such was a realignment.
In 1976, Northerners showed that they would not vote for a southern moderate populist like Jimmy Carter, and given a second chance to do so in 1980, didn't. In 1992 they voted for Bill Clinton, and did so also in 1996. In 1976 Carter won the South -- and lost it due in part to bad luck, but also due to the rise of the Religious Right that adopted Ronald Reagan as one of theirs. In 1992 and 1996 Clinton split the South. The 1992 and 1996 elections show that a southern moderate populist can split the white vote and win an overwhelming majority of the black vote to win statewide elections for Governorships and Senate seats.
In 2000 George W. Bush offered an ostensibly-moderate image that belied the Hard Right character of his ideology. The North and Far West generally rejected his agrarian-Christian fundamentalist-corporatist appeals, but enough of America went consistently for it that he could win re-election in 2004 against a former Tennessee Senator who had made too many compromises with liberalism for white Southern tastes and against a very liberal US Senator from Massachusetts. Between 1992 and 2004, Democrats consistently won 247 of the electoral votes necessary for outright victory in the Electoral College from the same states and the District of Columbia; in 2000 they got only 17 more electoral votes, and in 2004 only 4 more.
In 2008, the Obama campaign began with an effort to consolidate support in States deemed certain to vote for the Democratic nominee (comprising the same 247 electoral votes that had voted for the Democratic nominee every time since 1996; all of those States and the District of Columbia voted by 10+% margins for Obama) and twenty-one electoral votes from States that had voted for Dubya once (Obama got 9% or higher percentages of the vote in those states). The strategy was simple: Gore'00+NH = Kerry'04+IA+NM = 264; add five electoral votes by any means and win (because state Congressional delegations would decide the election to the advantage of Obama). There were plenty of targets scattered across the country: Nevada (5), Colorado (9), Missouri (11), Indiana (11), Ohio (20), Virginia (11), North Carolina (15), and Florida (27), all of which would decide the election. The Republicans had to defend every one of them. We know the result.
Obama got crushed in a bunch of states that Clinton won at least once -- demonstrating that Barack Obama was definitely not Bill Clinton and was unable to win the states that a Southern moderate populist could win. Who runs still matters, hardly demonstrating that there was a realignment in politics. Obama won three Southern states, though -- Virginia and North Carolina (which Clinton had never won) and Florida, which is Southern in the sense that Hawaii is southern -- only in latitude. Realignment implies that some states that used to be reliable for one Party have become reliable for another. Could someone like Bill Clinton have won much as Obama did? Probably not; he would have won a different set of States.
Very good analysis.
