Crusader isn't a word usually applied to either President Obama or Mitt Romney, the presumptive Republican presidential nominee. Close associates describe both men as reserved, analytical, and empirical, usually happier when diving into a spreadsheet of data than a room of outstretched hands. Neither is entirely trusted by his party's ideological vanguard.

And yet their competition for the White House in 2012 could produce the greatest issue divergence between the parties in decades. "In ideological terms, this is probably the starkest contrast we've seen since at least 1984 between Ronald Reagan and Walter Mondale," said veteran Democratic strategist William Galston, the issues director for Mondale in that campaign and now a senior fellow at the Brookings Institution. Others reach back even further: On the central issue of the role of the federal government in American life, the distance between Obama and Romney may be wider than the differences between any two nominees since Democrat Lyndon Johnson and Republican Barry Goldwater in 1964.

In 2012, few people are likely to complain that this election is offering a choice, not an echo. "We don't have "˜small ball' this year," says Peter Wehner, a senior fellow at the Ethics and Public Policy Center who was director of strategic initiatives in George W. Bush's White House. "We're not arguing over midnight basketball. It's not going to be a convergence of views. It's going to be a philosophical clash."

In this issue, National Journal begins a 13-part series that will examine that clash across a broad array of issues. The articles, starting with this week's look at the economy, will explain Obama's and Romney's principal proposals on key issues, list their major advisers, and explore how their positions compare with the views of key constituencies in each party.

Even with many details yet to be clarified, it's already evident that the rivals will offer the nation inimical directions. As our stories will explore, they diverge on, among other things, taxes; the structure of entitlements such as Medicare and Medicaid; the future of Obama's health care and financial-services reforms; energy; the environment; social policy; foreign-policy challenges from Russia to Iran; and the House Republican budget drafted by Rep. Paul Ryan, R-Wis., which Romney has praised lavishly and Obama has denounced as "thinly veiled social Darwinism."

These differences are more pronounced than might have been predicted nearly a decade ago when Romney was emerging as the deal-making centrist governor of reliably Democratic Massachusetts, and Obama was rising as an eloquent advocate for reconciliation in an America rigidly divided between red and blue. Yet in the years since, each man has been carried by the polarizing current separating Republicans and Democrats. For most of his career, Romney did not meet anyone's definition of a movement conservative; yet in adopting reliably conservative positions in this campaign he has aligned himself with the preferences of that camp on virtually all questions. Obama has disappointed liberals more frequently (on issues such as the public option in health care) but, overall, he has challenged the prevailing views in his party much less often than Bill Clinton did and has deferred more to the demands of his congressional leadership.

The magnetic pull on Obama and Romney to conform to his party's dominant views largely explains why these men whose instincts may be pragmatic are generating such a stark contrast in direction. In the increasingly parliamentary nature of modern American politics, where each party has grown less tolerant of dissent, both are unavoidably running as part of a team. And in this environment, even a president has only limited leeway to reconfigure his team's priorities and preferences.

"For different reasons, Obama as president and Romney as candidate have both had to respond to both the mobilized bases of their political parties and to the senior congressional leadership of their respective parties," Galston notes. "If Romney were painting on a blank slate, would he have thrown his arms around the Ryan budget the way he did? I don't think so. Similarly, I think that Obama has been made a more partisan and ideological president than he wanted to be, not only by the nature of the congressional Republican opposition but also by the center of gravity of his own party."

Romney faces more of this centripetal pressure than Obama does. After engineering a substantial expansion of Washington's role during his first term, Obama has revealed a second-term agenda that thus far appears defensive, aimed more at preserving than extending his initial advances. Romney, like Obama in 2008 but perhaps even more so, is the one who is promising vast change — a retrenchment of Washington that would topple pillars not only of Obama's program but also of the Great Society. The most ardent activists on both sides fear that each man in 2013 would be more willing to compromise with the other party than his proposals today suggest. But the starting point of any negotiations between the parties after the election will be the positions that each side adopts before it, and it is those contrasting commitments that National Journal will document in the weeks ahead.

William Friedman contributed contributed to this article