The 2008 Democratic primary battle between Senators Barack Obama and Hillary Rodham Clinton, which concludes Tuesday with contests in Montana and South Dakota, has developed such a reliable story line that pundits can recite it in their sleep.

The first black and first woman with solid chances to win the White House have nearly split the Democratic Party, with his coalition of young voters, affluent liberals and blacks against her coalition of women, older voters, Hispanics and working-class whites. Mr. Obama appears on track to a narrow nomination victory if he can win over just a small fraction of the roughly 200 superdelegates who remain undecided.

But that numbing familiarity cannot obscure what makes this nomination fight singular. In its cost, duration, competitiveness and breadth of citizen involvement, it stands alone in the history of American presidential politics.

“We’ve had higher rates of participation, not just by voting but by volunteering and giving, than any other,” said Michael J. Malbin, executive director of the Campaign Finance Institute and a onetime Congressional aide to Dick Cheney when he was a congressman. “And there’s nothing that’s remotely close.”