This whole debate about whether 2008 was or was not a ‘realigning’ election is rather silly.
Since the turn of the last century, there have been 11 cases in which the presidency changed parties: 1912 (Wilson), 1920 (Harding), 1932 (Roosevelt), 1952 (Eisenhower), 1960 (Kennedy), 1968 (Nixon), 1976 (Carter), 1980 (Reagan), 1992 (Clinton), 2000 (Bush), and 2008 (Obama). In 9 of the 11 cases, the party winning the presidency had also made substantial gains in the Congress as compared with four years’ earlier (although not necessarily as compared with two years’ earlier). The two exceptions were the last two party changes before Obama: Clinton in 1992, when the Democrats were pretty much treading water in the Congress, and Bush in 2000, when the Republicans were doing likewise.
What ultimately distinguishes the elections that are considered to have been realignments is the efficacy of the governance of the rising party, rather than the force with which said party took office. Ronald Reagan and FDR, famously, had coattails — but so did Warren G. Harding, who brought the Republicans a net gain of 123 (!) seats in the House in 1920. One might likewise have been tempted to consider the combination of the Democrats’ landslide in the 1974 midterms and Jimmy Carter’s ascendancy in 1976 a ‘realignment’. Reagan and FDR, however, were effective Presidents, whereas Carter and Harding were not, quickly managing to relinquish most of what they had gained. Barack Obama, perhaps, may be the first President since Reagan in 1980 to have an opportunity to realign the country; whether or not he’ll do so is another matter.
(As to the two exceptions I discussed earlier: I think you can argue, in essence, that Bill Clinton’s election was something of an historical accident, a correction in the long bull market for conservatives that ran from 1980 through 2006. And Clinton governed from the center, arguably accomplishing more for conservatives during his presidency than he did for liberals, ranging from the Defense of Marriage Act to NAFTA to welfare reform.)