This year's U.S. presidential race is considered a tossup by both sides, and unforeseen events could prove determinative: contagion from the European economic crisis, war or terrorism.
The largest imponderable is the economy. With a Republican- controlled House that isn't eager to help a Democratic president in an election year, even a mild stimulus is a non-starter. Any small impetus provided by Chairman Ben Bernanke's Federal Reserve is beyond the control of politicians.
There are some foreseeable milestones, however, that will also help shape the outcome of the Nov. 6 presidential election. These include:
June 28: Give or take a few days, this is when the Supreme Court hands down its decision on President Obama's health-care law. More than a few Republicans see this as a win- win situation. If the measure is retained, the Republican base will be energized for the general election; if it isn't, Obama's signature first-term achievement will be undercut.
If the law is upheld, the campaign will stress the law's unpopular positions and the need to "vote in November to change that," says Ed Gillespie, a senior adviser to the likely Republican presidential nominee, Mitt Romney.
"If it goes out the window, people will feel like we've wasted two years we didn't have to waste when the president should have been more focused on job creation and the economy," Gillespie says.
Obama strategists are less upbeat and more nuanced. If the law is upheld, they plan to stress its popular elements, a challenge they've failed to meet for the past two years. If it's overturned, there will be a temptation to attack both the court's decision as driven by ideology and to put Romney on the spot as to what he would do.
"There are grounds, polling shows, to take on the decision; people think this is an exceptionally political court," says Geoff Garin, a Democratic pollster. "And there will be a downside for Romney. It begs the question of what he would do next. He really doesn't have any answers."
This month, the high court also is expected to rule on Arizona's anti-immigration law. The politics here may slightly favor the Democrats: If the measure is upheld, the court's decision will energize Hispanic voters who are likely to vote overwhelmingly for Obama.
Aug. 27-30: The Republican National Convention will help set the tone for the general election. Successful conventions, such as 1988 for Republicans and 1992 for Democrats, launched winning candidates. Conversely, messy conventions - the Democrats' in 1980 or the Republicans' in 1992 - proved impossible to overcome.
In Tampa this year, the Republicans will bash Obama as a failed president who is in over his head. They'll also try to shed their image as a narrow club of rich guys by trotting out struggling Americans with whom to empathize and by highlighting more diversity than commonly seen at Republican gatherings.
On the last night, in his acceptance speech, the nominee has two challenges. He needs to show he's plausible, not scary; that should be easy. The other is to offer a credible, hopefully inspiring vision to the country; that'll be hard.
"A presidential election campaign is a referendum on the record of the sitting president," Gillespie says. "But it's more than that." An inspiring vision of any kind would be an addition to Romney's playbook.
Sept. 4-6: The Democrats are holding an abbreviated convention in Charlotte. Aside from funding concerns, there is also the threat of rain on Sept. 6, when the president is scheduled to give his acceptance speech at an outdoor stadium. (It rained in Charlotte last year on Sept. 6).
The tenor of these three days is no secret. The first two will feature Romney-bashing: his private-equity past, his Massachusetts record, how he will return the U.S. to the George W. Bush days, etc...
"They're going to remind people that our guys aren't loved," says former Rep. Tom Davis, R. "There will be some Rick Santorum and what Mitt Romney said in the primary."
Then the president, ideally under a clear sky, will try to offer an economic vision that so far has eluded this campaign: "There is need for more clarity on our side on the economic message," Garin says.
The Democrats are counting on Obama to change that on Sept. 6 by framing the race as a fight about the future. "This should be his bridge to the 21st century," says Tad Devine, a top Democratic strategist who ran Al Gore's presidential quest in 2000. "The president needs to lay down a real marker of where to take the country."
Oct. 3: The first of four national debates is held. A review of the 10 presidential cycles that featured debates suggests these have minimal impact on the final outcome; exceptions were the initial John F. Kennedy-Richard Nixon debates in 1960 and Ronald Reagan in 1980.
Nevertheless, there will be lots of hype and this affords each candidate a chance to hold his opponent accountable: Romney will be able to call on Obama to explain the poor performance of the economy over the previous four years; Obama can highlight his opponent's many policy reversals and support for some hard- right positions.
Nov. 2: The unemployment data for October is released. Voters' sense of economic security will be well formed by the eve of the election. Yet with dueling messages - Obama claiming the economy is improving and Romney saying it isn't - a highly publicized jobs report four days before the election could be psychologically important.
The focus will be on whether the trend is generally positive, with unemployment moving down a little, or basically negative, with joblessness increasing.
If the rate is in the neighborhood of the high sevens, or below 8 percent, "people will start saying maybe things are getting better," Davis says. "If it's going back up, it may be over." Last month, the jobless rate rose to 8.2 percent from 8.1 percent.
Albert R. Hunt is Washington editor at Bloomberg News