The intensity of the initial skirmishes in the campaign between President Obama and Mitt Romney underscores a new reality about presidential politics. What happens in the months before Labor Day and the candidates’ debates in the fall will shape the race and, if history is a guide, determine who wins in November.

The next 60 to 90 days may be among the most important of the general election. Obama and Romney will attempt to frame the issues on terms most favorable to their candidacies. Their campaign teams will try to define their opponent as negatively as possible. Both sides will put in place the money and machinery needed to spread their messages and turn out their voters on Election Day. Mistakes will be costly.

“There’s a pretty broad recognition that you can’t wait,” said Mark McKinnon, who oversaw the media in President George W. Bush’s 2000 and 2004 campaigns. “Certainly significant events may happen to reshape the race, but the fundamentals set in pretty early. You’ve got to use all the resources you have to blitz on every play and get every advantage you can as early as you can.”

What used to be seen as a transitional period between the nomination season and the general election has disappeared, leaving little margin for error, particularly for the challenger and his team who are coming off a long primary fight that has left them exhausted and the campaign’s resources depleted.

“There’s no such thing as a fall campaign anymore,” said Steve Schmidt, who was the chief strategist for Sen. John McCain’s 2008 general election campaign. “Once the nomination is sewn up, a presidential campaign is a continuous enterprise. The fall campaign is fundamentally about executing on the platform you build over the spring and summer. Wasted time is hard to make up.”

The past week was emblematic of the pace and demands of today’s general election campaigns. Romney’s last major opponent, former Pennsylvania senator Rick Santorum, suspended his campaign last Tuesday, but there was no time for celebration or relaxation in Romney’s Boston headquarters.

In the days since, Romney’s team has engaged in a heated exchange over working mothers. The candidate delivered the first of what are likely to be several speeches aimed at drawing contrasts with the president. Obama’s campaign moved aggressively to try to keep Romney on the defensive, while running as quickly as it could away from Democratic strategist Hilary Rosen’s sharply worded criticism of Romney’s wife, Ann, as a woman who had never worked “a day in her life.”

Labor Day was once seen as the official kickoff to the general election, a characterization that seems quaint in this era of round-the-clock politics and hyper communications. In fact, early September may be the moment that signals to the country which candidate is likely to win.

It is overwhelmingly the case that the candidate who has led just after Labor Day has gone on to win the election. The fact that the conventions are now held around Labor Day, rather than much earlier, means that the first polls taken after the post-convention bounces have dissipated will be key indicators as to how the race will go.

Among the exceptions: Ronald Reagan trailed Jimmy Carter in a mid-September 1980 Gallup poll and went on to win an electoral landslide. Al Gore led George W. Bush narrowly in an early September 2000 Gallup survey. He won the popular vote but not the presidency. But in virtually every other case dating to 1952, the leader in the Gallup Poll around Labor Day went on to win.

Four years ago, McCain led Obama briefly in mid-September, but that was more a reflection of the boost he got from his convention. But those polls were an anomaly in a campaign in which Obama always appeared in control. Through much of July and August that year, McCain’s campaign team feared that the election was already lost.

An adviser to McCain at the time recalled a conversation with the candidate shortly after Obama had returned from his successful overseas trip that included a speech before an estimated 200,000 people in Berlin. “He said are we ever going to be able to catch this guy,” said the adviser, who declined to be identified in order to share the details of a private conversation. “He was personally very depressed at this point. He felt he [Obama] was running away with it before we could even engage him.”

All general election campaigns include signature moments that long have been seen as helping to shape the outcome. For both Obama and Romney, the speeches they deliver at their respective national conventions will give them a chance to define the choices before huge national audiences. The presidential debates will offer the public a last look at the two nominees side by side, though they have only occasionally been seen as the decisive moments of the campaigns.

Romney will help define himself further with the choice of a vice presidential running mate. The way he manages that process and his choice of a running mate will affect public perceptions of him as a possible president. Obama will command the stage in his official duties, often a decided advantage in a reelection contest.

Most of those events will come in the second half of a campaign that will go on for almost seven months. But long before they take place, the skill and aggressiveness of the two candidates will help to lock perceptions of voters into place.

Outside events also will play a role in this early phase of the campaign. The Supreme Court is expected to rule sometime in June on the constitutionality of Obama’s health care law. Wisconsin voters will decide early that month whether to recall Republican Gov. Scott Walker. Overseas, tensions with Iran could boil over into an international crisis.

Campaigns can’t control those outside events, but they already are doing what they can to affect the race. Obama appears to be taking a page from Bush’s 2004 playbook by moving as quickly as possible to define Romney negatively.

“We made a decision that we needed to and wanted to frame the election on our terms and to do it as early as possible and as forcefully as possible,” McKinnon said of Bush’s 2004 campaign. “Their nominee was physically exhausted and depleted in resources and that was a perfect time to strike.”

Tad Devine, who was a top strategist for Democratic candidate John Kerry, said those early days were a huge mismatch between a Bush campaign that had, by his recollection, $100 million in the bank and a Kerry operation that ended the primaries with only about $2 million in cash on hand.

Devine said the Kerry campaign tried to counter with a big online fundraising drive, quickly launched a crash research project to figure out the terms of the election and aired some television ads to try to counter Bush’s television blitz. “We knew we didn’t have three months,” Devine said. “We knew we had a few weeks or we would get plowed under.”

It was during that early phase, however, that Kerry made a critical mistake — saying he had voted for funding the war in Iraq before he voted against it — that gave the Bush team fresh ammunition to drive home its favored message, that Kerry was a flip-flopper.

Obama’s campaign, like Bush’s in 2004, has had months to prepare for a general election contest against Romney. Obama advisers assumed from the start of the campaign a year ago that Romney would be their likely opponent and have been attacking him for months, though not with much paid advertising. That is expected soon.

Obama’s campaign also has stockpiled its cash for this moment. That puts Romney, who needs to replenish his bank account, at an obvious disadvantage at the start of the general election. But he has one asset that Kerry could not count on eight years ago, which is the existence of super PACs that can help to provide cover for the Republican candidate as his campaign seeks to replenish its bank account and expand its fundraising capacity for the general election.

Romney also has to do two things at the same time: unite a party whose conservatives still regard him with some suspicion and move to the middle to make up ground lost during the primaries. His deficit among women is particularly large. The speed with which his campaign began to address that problem since Santorum dropped out showed not only that his team sees it as a problem but also that the campaign is prepared to act immediately to try to correct it.

All of that explains the sense of urgency in Boston and Chicago. “The loser of this period can still go on to win the election,” Devine said. “But the loser of this period is more likely to lose the election.”