In what could be a pivotal day for the Democratic and Republican White House hopefuls, voters coast to coast will head to the polls in 24 states and American Samoa Tuesday.
Super Tuesday is virtually a national primary day, and some of the biggest prizes of the primary season -- California, New York, Illinois, New Jersey, Missouri and Georgia -- are up for grabs.
More than four-fifths of the 2,025 delegates needed to clinch the Democratic presidential nomination and more than 1,000 of the 1,191 necessary delegates on the Republican side are at stake.
Tuesday's results are more likely to be decisive to the contest among GOP nominee's Sen. John McCain of Arizona, former Massachusetts Gov. Mitt Romney and former Arkansas Gov. Mike Huckabee due to how the GOP allocates delegates.
Unlike Democrats, who usually split delegates by a proportion of the popular vote, the GOP will allocate delegates in most primary states on a winner-take-all basis -- either by congressional district or by statewide vote. That means that big-state wins by McCain, who leads national GOP polls, have the potential to propel him to a towering lead over his rivals.
While last-minute polls indicate Romney has gained ground against McCain in California, Romney is likely to split races with Huckabee in the South, Republican strategist John Feehery said.
"In the Northeastern states and Midwest, I think John McCain is going to win," Feehery said. "I think it's really hard for Mitt Romney to get this nomination."
On the Democratic side, the surviving contenders -- Illinois Sen. Barack Obama and New York Sen. Hillary Clinton -- are likely to split the delegates more evenly, Democratic analyst Peter Fenn said.
"I think that Obama clearly has an advantage in those seven states that are doing caucuses," Fenn said. "I think she's got to carry the big states that she had planned on. California is clearly up for grabs now ... Unless one or the other gets annihilated, I think they go on to many more Tuesdays."
In a CNN/Opinion Research Corp. poll out Monday, the two Democratic front-runners were virtually tied.
Obama, who trounced Clinton in January's South Carolina primary, garnered 49 percent of registered Democrats in Monday's poll, while Clinton trailed by just 3 percentage points.
With a sampling error of plus or minus 4.5 percentage points, that margin is too close to say which Democrat is leading.
The states holding contests this week include some of the biggest media markets in the country, and the Democrats have poured millions into television ads in those states.
The Campaign Media Analysis Group, CNN's consultant on campaign ad spending, estimates that Clinton and Obama had spent about $18 million combined in Super Tuesday states by the end of January. Obama even bought time for a television spot during Sunday's Super Bowl.
Republicans had spent only $2 million before the first week of February, CMAG reported.
Clinton and Obama made last-minute swings through the Northeast on Monday, with the former first lady returning to her touchstone issues of health care and the economic concerns of the middle class.
"You know, the middle class is under tremendous pressure," she told participants in a roundtable discussion in New Haven, Connecticut. "It is everything. The cost of everything is going up. And even if you make what used to be considered good wages, they don't cover the increase in costs, in everything from energy to health care."
Obama, meanwhile, emphasized his ability to attract independents and Republicans in an appearance in East Rutherford, New Jersey -- the home of the Super Bowl-winning New York Giants.
Obama was introduced by actor Robert De Niro and joined on stage by Sen. Edward Kennedy. Kennedy is a Massachusetts Democrat and a fan of the New England Patriots, the team the Giants beat Sunday night.
"I have said repeatedly that this campaign is about bringing people together. And for me to be able to bring a Patriots fan to the Meadowlands the day after the Super Bowl is like bringing the lion and the lamb together," Obama said. "We can bridge all gaps and all divisions in this country."
In the Republican contest, the two front-runners, McCain and Romney, have engaged in some bitter exchanges over each others conservative records.
Romney spent Monday trying to chip away at McCain's lead by positioning himself as the conservative alternative to the Arizona senator. McCain has infuriated the GOP's activist base with his support for immigration reform and campaign-finance reform, while Romney has blasted him for voting against President Bush's 2001 tax cuts.
"We disagree on a number of issues, and if you look at Senator McCain's position on a number of issues, you have a hard time distinguishing him from Hillary Clinton and Barack Obama," Romney told supporters in Georgia.
He acknowledged that he is "definitely the underdog compared to Senator McCain" -- but added, "The nice thing about being the underdog is that we typically win."
Not to be outdone, McCain has been attacking Romney for raising taxes, while refuting claims he's not conservative enough.
"I want to tell you, and look you in the eye, my friend, as president of the United States, I will preserve my proud conservative Republican credentials, but I will reach across the aisle to the Democrats and work together for the good of this country," McCain said at a rally in Boston, Massachusetts, Monday.
"That's what you want us to do," he added.
Romney suggested Sunday that Huckabee, who has struggled since winning the January 3 Iowa caucuses, get out of the race because he was splitting the conservative vote. But Huckabee said Romney's assumption that Huckabee supporters would turn to him was "presumptuous and even arrogant."
"If people are looking for someone who is an authentic conservative, who didn't just make up that platform to run for president, then I'm going to be their logical choice," Huckabee told CNN's "American Morning."
On Monday McCain picked up the endorsement of former New York Gov. George Pataki and already has the support of former New York Mayor Rudy Giuliani, who dropped out of the GOP race after last week's Florida primary.
McCain's endorsement by Florida Gov. Charlie Crist is credited with helping the Arizonan win that state's primary last week.
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"Never doubt that a small group of thoughtful and committed citizens can change the world. Indeed. Its the only thing that ever has." Margaret Mead