With the matchup between Sen. John Kerry and President Bush now assured, the stage is set for what observers believe could rank among the most classic - and potentially divisive - confrontations in decades.
After a period in which the two major parties often seemed to blur their differences as much as air them, these two candidates promise to offer clear-cut differences on a range of major issues, from foreign policy to trade agreements, from taxes to the death penalty.
Stylistically, the contrast may be equally stark, pitting an intellectual former war hero from the Northeast against a plain-spoken Texan who has never seen combat but has led the nation to war. The matchup makes it likely that the fall election will be close, and could further polarize an already divided nation. It may also have a profound effect on the direction of the country, magnifying the stakes for both sides.
Already, many observers see this campaign as a clash of historic proportions. "You probably would have to go back to '64, when Goldwater was running against LBJ [to find] such a basic, fundamental choice between liberals and conservatives," says presidential historian Haynes Johnson.
Others go back even further: "I don't think we have seen so clear-cut a choice between two candidates since FDR ran against Herbert Hoover in 1932," says presidential historian Robert Dallek.
Both candidates are working to present their opponent as far out of the mainstream. The Bush campaign has already begun scouring Kerry's voting record, highlighting votes he cast against defense systems and portraying him as weak on national security. Similarly, they're casting Kerry's past positions on cultural matters - such as his vote against the Defense of Marriage Act, which banned federal recognition of gay marriage - as far to the left of where most Americans stand.
Bush is also working to portray Kerry as indecisive and prone to flip-flopping, referring in recent political speeches to the Massachusetts senator as having two positions on everything.
The approach is similar to the one used against Al Gore in the 2000 election. But Republicans believe it may be even more effective against Kerry, who spent much of the primary campaign struggling to explain the nuances of his position on the Iraq war - and who, more recently, has had some of the same problems in clarifying his stance on gay marriage.
"He's similar to Mr. Gore in his apparent willingness to say anything, and he is well to the left of Mr. Gore on most of the issues on which he's voted," says Whit Ayres, a GOP pollster based in Georgia.
But Bush's own image has shifted, too, since the 2000 election, when he effectively positioned himself as a moderate "compassionate conservative." On the trail, Kerry portrays Bush as an ideologue who has brought the country down an extreme path by nearly every measure. He criticizes Bush's foreign policy as "reckless" and "inept," and attacks the president for giving tax breaks to the wealthy and big corporations rather than standing up for average Americans.
And while the Bush campaign may portray Kerry as evasive, Democrats are firing back at the president's credibility - hitting him, particularly, on the failure to discover weapons of mass destruction in Iraq. These attacks have already taken a toll, with polls showing more members of the public questioning Bush's trustworthiness.
Current polls show Kerry tied with or beating Bush. But Bush has only just begun to campaign - airing his first ads of the season this week - and will have record sums of money at his disposal over the next eight months. Currently, Kerry remains largely unknown to many voters, making the fight to define him critical over the next eight months.
Still, both sides agree the election will be a referendum on Bush's four years in office, with the economy and the iraq war dominating the debate, though cultural issues such as gay marriage could also play a key role. And most see the contest coming down to a handful of battleground states - with the rest of the country immovably fixed in "red" or "blue" territory. The most critical states may be those where cultural leanings tend to give Bush an edge, but where the loss of manufacturing jobs has caused serious economic pain and could ultimately boost Kerry.
"There are a few states that are really, truly in play," says independent pollster Del Ali. "It will come down to the economy in New Hampshire, West Virginia, Ohio. If voters are ambivalent, the bottom line is, Kerry wins the race. If voters are confident about [the economy], Bush ekes it out."
To some, the fact that so few states are likely to be competitive may work to lessen the ideological contrast, as both candidates move to the middle to try to win over the swing voters who could determine the overall outcome.
"The stakes really come down to the middle," says Democratic pollster Peter Hart. Both parties have to work at turning out their bases, he acknowledges, but ultimately, "it will be the same kinds of counties and voters that decide it that decided [every recent election]."
But others argue the two candidates are already locked in a clash that will inevitably be polarized around cultural and policy differences - and personal style.
The differences can be noted "just by looking at geography," says Mr. Ali. "Kerry is from possibly the most Democratic state and Bush from the most Republican state."
Indeed, many see the contest as a replay of the 2000 election - only more contentious. The fact that Kerry won the Democratic nomination "defines the Democratic Party as liberal on virtually every aspect of American politics," says Merle Black, a political scientist at Emory University. On the other hand, he adds, Bush "is not as popular as he was two years ago."
• David T. Cook contributed to this report from Washington.