Now, though, it's time for both politicians to start maximizing their appeal to the broad electorate, a task Obama had the luxury of starting early as the incumbent without an intra-party struggle to settle. And as they compete for that middle ground, the essential differences between them may become harder to see.
Those differences surely exist. Obama and his Republican challenger are offering voters a distinct choice on taxes, a sharp disagreement over health care and a classic ideological divide on social issues that neither candidate seems eager to talk about. So, too, Obama has shown he believes in the power of the purse -- or the power of debt -- to right an economic downturn in ways that Republicans find hard to swallow.
Take taxes. Romney and Obama are at odds over whether low taxes on the wealthy help fuel the engine of economic growth or are an unfair giveaway to people who don't need it.
That's far more than a debating-society point. Despite the substantial tax cuts Obama has supported since becoming president, he wants to push even more than in 2008 to raise taxes on the rich and on companies that outsource jobs. Romney wants lower rates for all incomes and no special tax penalties on corporate behavior.
Romney vows to try to roll back Obama's health care law if the Supreme Court doesn't do it in its pending ruling on the case, and to set a different course that lets states drive policy on that front.
Should he succeed, it would be a massive uprooting of one of the most significant reconstructions of social policy in generations. This from a man who, as Massachusetts governor, pioneered the approach Obama adapted for the nation at large.
Although presidential candidates rarely are explicit about it, they have a hefty interest in reworking the ideological balance of the Supreme Court, now with the slimmest conservative majority, should they get that chance.
Romney's opposition to abortion rights can't go very far when those rights were affirmed long ago by the court, but that could be a different story with the addition of another conservative or two. Obama's health care law would not be in judicial jeopardy if he'd been able to nudge the court left.
Both have records that defy easy labels.
Romney governed pragmatically in Massachusetts. He says he had no choice in a state so dominated by the opposing party. That record, along with his flips on some issues, has fed the perception that he's a man of the middle, not necessarily a terrible place to be in a general election. But if he's gone through the GOP race as a moderate in conservative's clothing, that's not to say he and Obama are indistinguishable on the big and little issues of the day.
Obama clearly believes in the value of regulation, despite efforts to roll back some burdensome rules. Romney just as evidently wants to cut them, despite assurances he's not looking to throw out the whole federal rulebook. Obama's environmental record is mixed, even disappointing, in the eyes of advocates, due to his aggressive posture on drilling and his failure to win a cap and trade law even from a Democratically controlled Congress.
But he still stands in contrast to an opponent who wants to open more protected waters for exploration, looks dimly on the potential of green energy, questions the science of climate change and blames environmental regulation for holding back the promise of American energy.
For all those Republican primary debates and Obama's time in office, there are blanks that need to be filled in for both.
Obama has not come up with a plan to rescue Social Security; his opponent has made more of a start on that by proposing to raise the future retirement age for full benefits by one or two years and to reduce inflation increases in benefits for wealthier recipients, while protecting the status quo for people 55 and over.
Romney's handling of any number of foreign policy crises cannot be seen until and unless they confront him in office, whereas Obama has a record to judge.
In any event, only so much can be read into their positions. The makeup of Congress after the election will determine whether a candidate's to-do list gets done, a reality often neglected when presidential candidates make bold promises.
Circumstance, too, will shape what gets done. Obama walked into a steep recession, with the financial sector, auto industry and housing market reeling. By necessity he became a manager on the fly, much as George W. Bush did when the terrorist attacks of 2001 changed everything for years to come.