For the pair of presidential candidates who must feel they have been campaigning forever — after all, it's been nearly a year since the opening Iowa Straw Poll— a milestone Sunday signals the final sprint.  One hundred days until Nov. 6.  "It's a signal we're turning the corner," says David Axelrod, President Obama's chief strategist, calling it a "psychological milestone." To mark the occasion, the Obama team plans more than 4,200 voter-registration and volunteer-recruitment events across the country Sunday, including "BBQs for Barack" in Ohio and Olympics-watching parties in Nevada.  Steve Schmidt, a top adviser to Republican John McCain in 2008, notes that years of preparations and calculations by Obama and Republican challenger Mitt Romney now face a rapid-fire final test. "People have been working on the Obama campaign since 2007, and this is the last race, the last campaign (for him). For Romney, a lot of those people have been working since 2006, 2007," he says. "And they remember it's 100 days to go."  "The last 100 days present a great opportunity because that's when voters are paying the most attention," says Jonathan Collegio of American Crossroads, a pro-Romney super PAC.  Much of the focus will be even tighter, on perhaps 10 hours within those 100 days: Romney's announcement of his running mate next month, the end-of-the-summer speeches delivered by Romney and Obama at the national political conventions, and the three presidential debates scheduled for October.  Some political scientists have devised academic models that downplay the significance of the campaign's final weeks in favor of broad political and economic forces. Alan Abramowitz of Emory University, for one, predicts the popular vote using just three factors, each already set: whether an incumbent president is running (yes), the sitting president's net approval rating in the final Gallup Poll in June (48% approval-46% disapproval) and the change in real GDP in the second quarter. The initial estimate of the nation's economic growth from April through June is due Friday.  His model — which accurately predicted the popular-vote winner in the past five elections — calculates that Obama needs second-quarter growth of 1% or higher to win a majority of the popular vote.  Here's another: In nine of the past 10 elections, the candidate leading in the Gallup Poll taken closest to 100 days out has won the White House. (The exception was 1988, when the 100-day mark came a week after the Democratic convention had given Michael Dukakis a bump. It also came two weeks before the Republican convention nominated George H.W. Bush.)  On Wednesday, with four days to go until the 100-day mark, Gallup's seven-day rolling survey put Romney at 46%, Obama at 45%.  There was a time when Labor Day marked the beginning of the general-election campaign after a summertime lull. Now, presidential battles have expanded in time and increased in intensity, changing their rhythm.  The political conventions have been pushed later in the summer, but the advertising barrage starts earlier as both sides try to shape positive impressions of their candidate and negative impressions of their opponent. There's no longer a summer break: This month, an estimated $100 million worth of political ads have aired in swing states.  The endgame has changed, too. The expansion of early voting and no-excuse-needed absentee voting means more votes are cast earlier. And this year, for the first time, jurisdictions are required to send absentee ballots to active-duty military voters by late September. Political scientist Michael McDonald of George Mason University estimates that a third of all votes will be cast before any polls open on Nov. 6.  "Our 72-hour campaign (to get out the vote) has become a 72-day campaign," says Sean Spicer of the Republican National Committee.  The campaigns have strategies in hand, ads in mind and travel schedules blocked — subject to change as key states become more or less competitive.  "The groundwork has been laid for the train going forward," says Bill Burton, a veteran of Obama's 2008 campaign who now helps run a supportive super PAC, Priorities USA Action.  To be sure, unexpected events could intervene. Days before the 2000 election, the disclosure of a drunken-driving arrest in Maine from a quarter-century earlier rattled George W. Bush's campaign. In 2004, Osama bin Laden released a tape in the campaign's final days in which he admitted for the first time that he was behind the 9/11 attacks, reviving fears of terrorism among some voters — a boost for Bush. In 2008, Lehman Brothers filed for bankruptcy protection on Sept. 15, stoking a financial crisis and, Axelrod says, putting the candidates' leadership on display in a way that clinched the election for Obama.  There are also moments we know to expect, and that are likely to matter, over the next 100 days. Here is a reader's guide to five of them.  In London on Friday, Michelle Obama will lead the official U.S. delegation at the opening ceremonies of the Olympic Games, typically an event marked by national unity and pride. Mitt Romney will be there as well, a reminder to voters of his success in turning around the troubled Salt Lake City Winter Olympics in 2002. Ann Romney will stick around to see the horse she co-owns, Rafalca, perform in the dressage competition.  The London stop is the start of a three-nation tour by Romney designed to show his potential as a credible commander in chief, meeting with foreign leaders and discussing international policy on a global stage. He is scheduled to meet with British Prime Minister David Cameron, Israeli Prime Minister Benjamin Netanyahu and Polish Prime Minister Donald Tusk. He'll deliver speeches in Israel and Poland.  Obama took a seven-nation world tour of his own at this point in the campaign four years ago — to Iraq, Afghanistan, Israel and elsewhere.  Romney outlined his foreign-policy case against Obama in a speech Tuesday in Reno at the Veteran of Foreign Wars Convention, a group the president had addressed the day before. "I pledge to you that if I become commander in chief, the United States of America will fulfill its duty, and its destiny," Romney told them.  History says the unemployment report for July, released at the end of next week, will do more to shape November's outcome than those that follow. That's because election-year perceptions of the economy begin to be firmly set in the summer — and the economy is driving this election.  "There comes a point at which judgment is formed as opposed to just an opinion, and it's hard to reverse a judgment," says Ed Gillespie, a senior adviser to Romney. "The last unemployment report was important, and the one in August will be important, too."  Consider 1992, when the elder President Bush was running for re-election as the nation was pulling out of a recession. The jobless rate peaked at 7.8% in June and was almost as high, 7.7%, in July. It steadily ticked down after that, to 7.3% by October, but Bush tried in vain to convince Americans things really were getting better.  Democratic challenger Bill Clinton ridiculed the president's reassurances in an attack ad that fall. He won in November.  This year, the October jobs report is scheduled to be released on Nov. 2, four days before the election. Other important measures of the economy's course are the GDP estimates — due July 27 for the second quarter (with revised estimates in August and September) and on Oct. 26 for the third quarter, from July through September.  Romney is likely to announce his running mate sometime during the two weeks after the Olympics end Aug. 12 — to avoid competing for attention with the Games — and before the Republican National Convention opens in Tampa on Aug. 27. The process is scrutinized in part as a sign of what kind of decision-maker the presidential candidate would be in office: deliberative or instinctive? Cautious or a risk-taker?  McCain's surprise choice four years ago of then-Alaska governor Sarah Palin, unveiled the day after the Democratic convention ended and three days before the Republican convention opened, underscored his trust-your-gut training as a Navy pilot. Romney's business past shows him as the deliberative sort who carefully would analyze the implications of various choices before acting.  Many news reports center on Ohio Sen. Rob Portman, an experienced Washington hand from a crucial swing state; former Minnesota governor Tim Pawlenty, a onetime competitor for the nomination who has become a top surrogate; and Louisiana Gov. Bobby Jindal.  The veepstakes has created a cottage industry for lawyers who specialize in vetting contenders, and it is catnip for cable TV commentators. How much it matters to voters isn't clear, however. The last vice presidential pick who clearly made a difference in an election was then-senator Lyndon Johnson, whose presence on the Democratic ticket was decisive in carrying his native Texas and putting John F. Kennedy in the Oval Office.  That was, of course, more than a half-century ago.  More than vice presidential picks, more than candidate debates, what matters in the campaign's final 100 days are their convention speeches, Christopher Wlezien of Temple University has concluded. He studied the issue for his forthcoming book, The Timeline of Presidential Elections.  Even with the advent of the Internet and the saturation of social media, some voters don't tune in to the election until the conventions, Wlezien says. The speeches give them a first look at a challenger who isn't familiar and a second look at a president who is. "It gets voters to take stock," he says. "Do I want to stay the course? Do I want a change?" The candidates' standings after the conventions have proved to be a reasonably reliable indicator of who will win.  Romney delivers his address Aug. 30 at the Tampa Bay Times Forum in Tampa; Obama, on Sept. 6 at Bank of America Stadium in Charlotte.  Both campaigns describe the moment as pivotal. "They are the biggest speaking platform the candidates are going to have to lay out their arguments," Axelrod says.  Gillespie says the speech "sets the frame for the campaign's homestretch. And it's a chance for people to see the nominee unfiltered, not in 30-second spots, not in characterizations or caricatures, but directly, and judge for themselves."  In 1988, the elder Bush solidified support in a convention speech emphasizing his determination to oppose tax hikes. "Read my lips: No new taxes," he vowed. That helped him win in 1988, although the fact he broke that promise as president contributed to his defeat in 1992.  If it's hard to find examples of vice presidential nominees affecting election outcomes, it's easy to find times when the presidential debates have defined the final weeks of a campaign. "Look, back as far as Kennedy-Nixon the debates have been pretty important," says Gillespie, a former GOP national chairman.  In 1960, at the first presidential debate, Richard Nixon's 5 o'clock shadow and sweaty mien contrasted with John Kennedy's ease; JFK won in November. In 1976, President Ford's gaffe ("There is no Soviet domination of Eastern Europe") was a distraction in the final days; he lost to Jimmy Carter. Ronald Reagan's confident debate demeanor in 1980 won over some voters, and his closing pitch — "Are you better off than you were four years ago?" — became a campaign mantra. He won.  Preparations already have begun in both campaigns for the debates. Obama has drafted Massachusetts Sen. John Kerry to play the role of Romney in his practice sessions. Four years ago, Portman was John McCain's foil in preparations for debates with Obama.  The Commission on Presidential Debates announced Wednesday that the first debate, in Denver, would focus on domestic policy. The second, in Hempstead, N.Y., will have a town-hall-style format. The third, in Boca Raton, Fla., will be on foreign policy.  "Those undecided voters typically wait until the debates are over to make their final decisions," says Donna Brazile, campaign manager of Al Gore's 2000 presidential bid. When the debates are over, Axelrod says, "people will make their own judgments."
After years of preparation, the president and his GOP rival have a few critical opportunities before Election Day to make their best case.