I suppose that, depending on which side of the political fence you're on (and that you actually have a clue regarding American presidential electoral history), this upcoming 2012 election will "fit" into a model represented by some past elections. For this article's sake, let's take the elections from 1968-2008: these are the ones I personally followed (since age 12).
To find a fitting model for 2012, we have to see, in previous elections, a sitting president looking to either be elected for the first time (Ford in 1976) or re-elected. In 1968, incumbent Lyndon Johnson had dropped out and his party's nominee Humphrey lost the election to Nixon. For this article's purposes, though, I will stick with an individual SITTING president running to stay in office. So the years 1968, 1988, 2000, and 2008 are out, still leaving quite a lot to examine.
In 1972, Richard Nixon was running for re-election while the opposition Democrats, instead of uniting around a more centrist candidate, chose George McGovern to run in the general election. The result was a near-record landslide for Nixon. In 2012, Democrats salivate while Republicans quake at the prospect of right-wing ideologue Rick Santorum securing the GOP nomination, portending a similar electoral outcome.
In 1976, appointed VP and eventual president Gerald Ford lost a very close election to smiling, ever-promising candidate Jimmy Carter. Carter tried to be all things to all people, while Ford carried the burden of his pardon to Nixon for any Watergate scandal crimes. The Republicans would like this to be a model for a November victory this year, but it can only be pulled off if more centrist candidate Mitt Romney can successfully pull away early from Santorum and start to court more centrist voters. Ford also had to suffer through a grueling party nomination campaign against Ronald Reagan, putting him at a disadvantage early in the general campaign. Obama is unopposed within his own party.
In 1980, it was Carter running for another term, this time against Ronald Reagan. And he probably would have stayed in office had the economy not tanked just before the election. This is another scenario that the Republicans would like to see, their claims to want the economy to improve notwithstanding. But like Ford in 1976 and again unlike Obama in 2012, the incumbent Carter had to endure a hard campaign within his own party, this time the opponent being Edward Kennedy.
In both Reagan and Bill Clinton's reelection campaigns in 1984 and 1996, respectively, the country was at peace, the economy was in very good shape, and the voters consequently endorsed the incumbent as representing the relatively successful status quo. Barack Obama and the Democrats see this as an ideal type of election, but in reality we are still at war and the economy isn't exactly booming, either. But to mitigate this, they will spin both situations as being on the rebound.
Another campaign that the Republicans derive some hope from is 1992 between incumbent George H.W. Bush, Bill Clinton, and temperamental (thank God he wasn't elected) business tycoon Ross Perot. But the way I see it, this isn't a very good model for two reasons. One, Perot probably helped Clinton more than he did Bush by siphoning more votes from the latter. Two, the Republicans had been in power for 12 years and the country wanted a change. In 2012, the Democrats have only been in the White House going on 4 years.
And now we come to the final election left to consider, and which I consider to be the most likely scenario for this year: 2004. Incumbent Republican George W. Bush, strongly embraced by his own party and vilified by the opposition, won a squeaker election against bland Massachusetts politician John Kerry in a very dirty, vicious campaign. For this election, the country was asked to stay the course, although we were in the middle of a quagmire of a war and an escalating national debt. Sound familiar...especially that part about the bland Massachusetts politician? The essential difference in 2012, besides the parties being reversed in their power/opposition roles, is that the economy is more of a burden to Obama while the war was more of a burden to Bush. But we'll see what really happens...