Looking at the historical record, 2008 isn’t a realignment sort of year. In the 20th century, there were three realignments that took place in Presidential election years — 1912, 1932, and 1980. What these elections had in common was that the President was highly unpopular, and he was running for reelection.
Having an unpopular President who is not running again just doesn’t produce the same sort of results. Maybe it doesn’t produce any result, if the last French election is any guide, where Chirac was unpopular but his party won anyway.
What about 1952, you might ask. Wasn’t that a realignment election? I suppose it may have seemed so at the time, with Eisenhower winning in a landslide and the Republicans taking both houses of Congress. However, the Republicans were closing in on majority status prior to 1952, had only modest gains in that year, and lost their majorities in 1954. Without a national hero on the top of the ticket, the Republicans didn’t make a lasting change to the electoral map.
Going back a ways, there was 1896. There weren’t any polls, of course, which saves Grover Cleveland some historical embarassment. There had been a period of exceptionally close Presidential races, which gave way to four straight Republican landslides. So, in terms of Presidential politics, it was a realignment moment. But there was a catch. The Republicans lost 48 seats in the House that year, making it a funny sort of realignment.
To see that there is no historical inevitability to a Democratic sweep in 2008, just consider for a moment what would be going on now if the Constitution didn’t require a President to be born in the US. It seems likely to me that Schwartzenegger would be the presumptive GOP nominee, and the favorite to win in November. It seems like a funny sort of realignment year that depends on the other side’s best candidate being ineligible to run.
There was a recent survey where people identified themselves as conservative, moderate, or liberal on social and economic issues. The double conservative bracket had 24% while the double liberals were only 9%. As long as Republicans go into every election with a nationwide ideological advantage of 15%, it’s not going be realignment, even if a sinking economy does bring the Democrats substantial gains.