Americans in a large patchwork of states will file into voting booths and caucuses on Tuesday in the closest thing the country has ever had to a national presidential primary.
The rules differ from state to state and party to party, but the combined outcomes could anoint one party's nominee, define a front-runner in the other party and test both parties' coalitions.
Tuesday could effectively decide the Republican nomination, if enough people jump on board John McCain's bandwagon or rally behind Mitt Romney. Polls suggest that they're leaning toward McCain.
In the Democratic race, they could break the tie between Hillary Rodham Clinton and Barack Obama and give one a real burst of momentum -- or deliver another split decision that sends them on to grapple state by state, delegate by delegate, into the spring.
"For Republicans, Super Tuesday may be the beginning of the end. For Democrats, it may only be the end of the beginning," said John Pitney, a political science professor at Claremont McKenna College in California.
The Super Tuesday contests, however, threaten to deepen divisions in both parties, racial and generational for the Democrats and conservative vs. moderate for the Republicans. That challenges candidates who want to turn out their base of support but still hope to rally the other side behind them for the fall campaign.
Tuesday's test is greater, perhaps, for McCain, who emerged as the Republican front-runner last week with his victory in Florida.
With leads in several delegate-rich states in which the winner takes all the state's delegates, the Arizona senator could easily win the majority of the 1,009 delegates available Tuesday.
He can't lock up the title, though. Even if he won every delegate in the 21 states with Republican contests, he would be 89 short of the 1,191 needed to win the nomination.
But taking a solid majority would give him undeniable momentum, draw a new infusion of campaign cash and probably secure more fence-sitting Republican officials.
"It looks like he's on his way to being the nominee," said Scott Reed, a veteran Republican strategist.
It also would create pressure on former Massachusetts Gov. Romney to show his donors -- or his family, whose inheritance he would be spending if he lent his campaign more money -- how and where he could overtake McCain. After Tuesday, another 1,133 delegates will remain available -- enough to grasp the nomination, but he'd have to take the vast majority of them.
A key question for Republicans is whether McCain will pull them apart -- supported by moderates but driving away conservatives. That chasm is growing wider by the day now that he appears the favorite.
Influential conservatives such as radio talk-show host Rush Limbaugh bitterly oppose him because of his votes against the Bush tax cuts, his outspoken criticism of torture as an interrogation technique for suspected terrorists and his advocacy -- since suspended -- of a path to citizenship for illegal immigrants who already are in the United States.
Sens. Clinton of New York and Obama of Illinois are locked in a drawn-out war of attrition for the Democratic nomination, sure to be fighting for delegates beyond next Tuesday no matter who comes out ahead on the primary season's biggest day.
The sheer size of the challenge could favor the former First Lady. She is still better known than Obama and holds a 7 percentage point lead in the latest national Gallup Poll, released Saturday.
But with a sizable bank account and lots of attention from his victory in South Carolina, Obama is prepared to go toe-to-toe with Clinton. And Obama benefits from party rules that award delegates according to the share of the votes each candidate receives. There's no winner-take-all for the Democrats, so Obama could still win a substantial share of 1,681 delegates up for grabs, even if Clinton wins most of the 22 states.
Clinton and Obama are basically the same on big issues: Both propose to expand health care to the uninsured, say they would raise taxes on the wealthy and say they would withdraw troops from Iraq.
They differ slightly, however, in how they say they would reach those goals.
Clinton stresses her experience in government, saying she knows how to pull the levers of power and push an agenda through Congress. Obama runs as a visionary who would transform politics by inspiring a broad new coalition united behind change.
In the process, they've played hardball politics that's attracting different coalitions. Hers is older women, downscale whites and Latinos. His is young people, affluent whites and blacks.
If they opened a racial divide in recent weeks -- the South Carolina vote in particular was divided clearly along black-white lines -- the two worked last week to heal the rift with a polite debate in California.
At least one Democratic strategist was optimistic that their feud didn't open a permanent racial divide.
"It scratched at the scab, but I'm not sure it peeled it off," Democratic pollster Mark Mellman said. "And it's not damaging for the long term."
The Associated Press contributed to this report.