Democrat Barack Obama and Republican John McCain this week plunge into the decisive phase of their sour White House battle, with the close race giving both a tantalizing hope of victory.
The presidential campaign has raged for a pulsating 18 months, and sucked up a billion dollars in campaign cash, while promising history and delivering large helpings of intense political acrimony.
Now, a frenetic two-and-a-half month sprint for the finish looms, with the ultimate prize still within reach for both campaigns.
"We are now entering one of the most intense political periods that we have ever seen," said McCain's campaign manager Rick Davis.
"We are jamming in a lot of major events, the selection of each candidate's vice president, their national conventions, the debates and election day all in an almost 10-week period."
Obama is throwing himself back into the crossfire after a vacation in his native Hawaii, with the most critical decision of his political career so far pending -- the selection of his running mate.
Unless he intends to make a surprise pick at the Democratic convention next week, Obama is expected sometime this week to introduce and campaign with his number two. Mystery still surrounds his, or her, identity.
Democrats meet in Denver in swing state of Colorado between August 25 and 28, in a convention choreographed to finally salve the wounds of the bitter primary fight between Obama and Hillary Clinton.
Republicans will embrace McCain in Minneapolis/St Paul, Minnesota, the following week, leaving the Arizona senator a little longer to wrestle with his choice of running mate.
Then the rivals will clash in presidential debates on September 26 and October 7 and 15, which analysts say could cement the decisions of voters before the November 4 election.
Going into this critical period, the race is remarkably stable, with Obama, 47, vying to become America's first black president, enjoying a steady yet small lead of a few points over McCain in national polls.
The rivals are also closely matched in battlegrounds like Colorado, Virgina, Missouri, Ohio and Florida which hold the key to the White House.
"This is competitive," pollster John Zogby told AFP.
"There are ups and downs with a slight advantage to Obama."
The McCain campaign is delighted to be hanging in close to Obama, in what is universally judged to be a bad year for Republicans.
"If anybody had asked us a couple of months ago ... whether or not we would like to be where we are today, which, from what we can tell, is a pretty even race ... we would have been pretty thrilled," Davis said.
The Obama campaign professes unconcern with national polls, betting on a massive turnout powered by the staggering grass roots movement its candidate has whipped up, now two million donors strong.
The tight race, remarkable in a year in which the Republican brand is badly tarnished, the economy limping and President George W. Bush highly unpopular, means the next few weeks could be decisive.
Despite fierce spats over foreign policy and the future of US troops in Iraq, and Afghanistan, it may be that the candidate who can best connect with voters on the number one issue, the deteriorating economy, emerges the victor.
Most polls say Obama has a slight edge here, but McCain compensates by being seen as the best potential commander-in-chief in an uncertain world.
Obama, who will close his convention by firing up 75,000 supporters in an unusual open-air keynote speech at a Denver football stadium, will be desperate for a "bounce" in opinion polls after his party's jamboree.
His failure to pull out a bigger lead on McCain suggest he has yet to 'close the deal' with voters, some of whom are concerned about his lack of experience.
The Denver convention will cap a staggering rise to prominence for Obama, who shot to fame during John Kerry's coronation as party nominee four years ago with an electrifying speech while still a mere state legislator.
McCain's battle to be presumptive party nominee is almost as impressive. Last year, the campaign of the 72-year-old, who would be the oldest president ever inaugurated for a first term, seemed doomed.
His effort was virtually bankrupt, and he was seen as a pariah by many in the Republican Party's core conservative bloc.
Now political observers are intrigued to see how voters will judge the visual dynamic of a face-to-face debates between McCain, a grizzled Vietnam War veteran and Obama, the charismatic face of a new generation.