On the night before the 2008 election, Barack Obama sounded a familiar mantra signaling the impending success of his once unlikely presidential bid. “We are one day away from changing America,” he told a cheering Virginia crowd.

The next day, by a solid majority of 8.5 million votes, Americans elected the 47-year-old Illinois senator, and he set out to implement the “big change” for which he contended the times were right. On Tuesday, HBO will premiere a film capturing the promise and optimism of Obama’s campaign. But a year later, the premise that the new approach of a new president could ensure such sweeping change no longer seems so simple.

A variety of factors — some predictable, others unforeseen — have changed the landscape and made it likely that any ultimate changes will be more incremental and less dramatic than Obama promised.

Here are the main ones:

• The economy and deficit proved far worse than they seemed last November.

That increased the focus on federal spending and meant less support for the expanded governmental programs Obama promised. Though President George W. Bush passed the Medicare drug benefit and launched a costly war in Iraq without paying for either, Republicans have harped on the cost of Obama’s health plan. This has made conservative Democrats more reluctant to support some initiatives, lest they suffer politically in 2010.

• Washington’s nasty partisan tone persisted, despite Obama’s vow to conduct a less acrimonious, post-partisan presidency.

Both sides deserve blame. Republicans say Obama’s reliance on congressional Democrats shows he wasn’t interested in their help, but many Republicans want to stop Obama just as they defeated Bill Clinton and the Democrats 16 years ago. In his new book, “The Clinton Tapes,” author Taylor Branch says former Senate GOP leader Bob Dole once told President Clinton that the opposition’s job is not making deals but “making the president fail, so he could be replaced as quickly as possible.”

• The health care fight dominating the debate has been more complex and time-consuming than Obama’s people imagined.

The fact that only a minority of Americans would benefit short-term has helped critics stir concerns among those who feared they’d mainly pay for the changes.

• Obama’s claim that a new approach to Washington would break the partisan gridlock sounded good but was never realistic.

The main reason for governmental gridlock is not how problems are approached but rather hinges on profound policy disagreements and the emphasis on scoring political points.

• Institutional barriers remain.

Though Democrats have the biggest Senate majority in 30 years and a solid House margin, the lack of Republican support and institutionalization of the need to repeatedly muster 60 of the 100 senators has complicated getting things done.

• Afghanistan clouds the landscape.

The Iraq withdrawal has been relatively smooth, in part because the public seems less interested. But Obama’s vow to follow through on promises to bolster the U.S. effort in Afghanistan added potential new costs and stirred heightened debate over the right course.

• Obama’s foreign policy initiatives have not yet produced concrete results.

They raised U.S. standing overseas and won him the Nobel Peace Prize, but progress takes time. Ultimately, Obama needs successes to mitigate the mentality that he talks a better game than he can produce.

Nevertheless, polls indicate Obama remains well-liked. Despite concerns about some policies, his job approval level has stabilized at about 50 percent or slightly higher.

More important, the only current public agenda remains the one Obama outlined in the campaign, thanks to GOP failure to articulate a positive alternative. His success or failure will ultimately determine the fate of his presidency.