Steven Stark makes an important point that I have oversimplified in the past.  Where I had cited the polling from past presidential races as evidence that the Democratic candidate consistently ran better early in the year, the reality is that the challenger against the incumbent party’s candidate is the one who should normally poll better at this point.  Stark explains:

At this point in the election cycle — before any fear of the unknown has set in — challengers are often running much better against their incumbent-party opponents. In 1988, Michael Dukakis had about a 10-point lead over George Bush (the senior and then-vice-president), only to lose by around eight — an 18-point swing.

Ditto in 2000. George Bush (the younger) had about a similar 10-point lead over Al Gore at this stage, only to see the lead shrink to nothing by Election Day.

In fact, that’s been the usual pattern. In 1976, Carter led Gerald Ford by 10 points in the spring, and even McGovern in the spring of 1972 found himself running roughly even with Richard Nixon (albeit with a potential George Wallace third-party candidacy in the mix). By November, the incumbent had surged considerably in both cases.

Even in 2004, John Kerry ended up doing worse in November than he had in the spring, at least according to the CNN/Gallup poll that gave him a five-point lead in April.

The only modern exceptions to this involved Bill Clinton, in 1992, and Ronald Reagan, in 1980. In both elections, the insurgents came from behind. But both faced notably different circumstances than Obama does.

You will say that there is no real incumbent this time.  So, does it matter that this is the first open election in over fifty years?  Even though McCain is not an incumbent, he is closely identified with the errors of the current administration and the incumbent party, which means that he may have all the disadvantages of incumbency and none of its strengths.  However, Stark suggests that McCain is helped by the fact that he is not the incumbent:

First, Clinton and Reagan got to run against unpopular incumbents. McCain is not George Bush — no matter how much Obama may try to tie the two together.

True enough.  McCain is much more reckless in foreign affairs and even more committed to amnesty.  His is a more refined, potent GOP-destroying agenda.  But that doesn’t seem to be the way many people perceive him, and the constant emphasis on how “moderate” he is supposed to be has clearly separated him from the damaged public image of mainstream conservatism.  Thanks to the reputation McCain has gained in the media, he will never receive the same scorn or criticism for taking positions identical to those of the current administration, and the one major domestic policy view he has that alienates conservatives is the one that wins him admiration from most journalists.  Journalists treat McCain the way some supporters treat Obama when he says something they don’t like. “Oh, he doesn’t really believe that–he’s just saying that to satisfy the rubes.”  (They might say the same thing about other Republicans, but they usually say it in an accusatory, rather than exculpatory, way.)  That will certainly be a difference with previous elections.  It is difficult to recall a case where the mainstream media was more devoted to building up the Republican nominee, but that seems to be happening this time.  He is their candidate, and they are his people. 

It’s worth noting that Obama was running at Dukakis and Kerry-like levels ahead of McCain in February, and has been losing ground ever since.  According to Rasmussen’s daily tracking poll, Obama now trails by 10 nationally, and their Missouri polling confirms the large McCain lead (53-38) that SUSA found earlier this month.  Even if the weak Obama state polling in a number of Democratic and swing states is a function of party infighting and will change once the nomination is determined, that simply drives home the point that a continued nomination fight on the Democratic side can only work to their disadvantage.  The ongoing contest is obviously driving up Obama’s unfav rating and reducing Clinton’s fav rating, so that whichever one the superdelegates select will be badly damaged come autumn.  There will be less time for the eventual nominee to shore up Democratic support, since the general election campaign will be starting within weeks of the convention in Denver.  Even if this is concluded in June, that means another two months of strife that benefit McCain.  Meanwhile, starting next week, McCain will have effectively been the nominee for two months, which does give the impression of being in the position of an incumbent.  Unlike Mr. Bush, however, McCain is not unpopular.  He is well-liked and viewed more positively than either of his would-be opponents.  After the last month, Obama may well unify the GOP against him more effectively than Clinton, and according to the current polls he fails to unite more than 75% of Democrats in many states.