Crusader isn't a word usually applied to either President Obama or Mitt Romney. Close associates describe each 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. On the central question of the federal government's role 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 squared off in 1964.

Even with many details yet to be clarified, it's already evident that the rivals will offer the nation diametric directions. Nothing crystallizes their disagreement more than the House Republican budget drafted by Rep. Paul Ryan of Wisconsin, which Romney embraced by selecting Ryan as his dynamic but polarizing running mate and which 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, the polarizing current driving Republicans and Democrats apart has carried each man.

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

The most ardent activists on both sides fear that each standard-bearer would be more willing to compromise in 2013 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 has documented in the pages that follow.

Ronald Brownstein