In seven state contests so far, John McCain has won no more than 37 percent of the vote, but with a strong assist from the Republican Party's winner-take-all system of apportioning delegates, McCain stands poised to dominate the field today and put himself on a clear path to the nomination.
At the same time, Hillary Clinton maintains a lead in polls in many large states, but thanks to the Democratic Party's system of allotting delegates by a complex formula based on percentage of the vote, neither she nor rival Barack Obama is likely to emerge from today's voting with a clear path to their party's nomination, even if one of them wins substantially more states.
The parties' contrasting systems of choosing candidates reflect both past battles and different underlying objectives: Among Republicans, a desire to rally the party behind the biggest vote-getter, even if the victory margin is small; among Democrats, a desire to maximize the participation of all constituencies, even if it delays the selection of a nominee. And with more than 1,000 delegates at stake on both sides, the disparate methods of electing delegates will control the destinies of both parties.
"I think the process for choosing delegates is probably going to shape the outcome of this election," said Lawrence R. Jacobs, director of the Center for the Study of Politics and Governance at the University of Minnesota. "The dif ferent outcomes we're going to face tomorrow are the result of these very different processes."
McCain's wins in New Hampshire, South Carolina, and Florida were small by percentage - 37 in New Hampshire, 33 in South Carolina, and 36 in Florida - but gained him enough delegates that several key challengers abandoned the race.
With the withdrawal of former New York City mayor Rudy Giuliani, who endorsed McCain, the Arizona senator grabbed a substantial lead in polls in both New York and New Jersey, two of the 11 largest states in the country. Both are winner-take-all primaries, and under the GOP system no one has an incentive to campaign there unless they think they can win.
Given McCain's big lead, both of his main challengers still in the race - former Massachusetts governor Mitt Romney and former Arkansas governor Mike Huckabee - have largely stayed away. Romney campaigned briefly in New York and not at all in New Jersey, shifting his attention to Western states including California, one of relatively few states that divide its Republican delegates by congressional district rather than statewide.
This week, McCain has campaigned with growing confidence in recent days, sensing a chance to deliver a devastating blow to both Romney and Huckabee. With McCain all but certain to reap big troves of delegates in New York, New Jersey, and many other northeastern states, Republican officials who had been reluctant to endorse him earlier are now clambering aboard his campaign, creating a growing bandwagon effect.
Yesterday, the McCain team added another slew of GOP leaders - Senator Norm Coleman of Minnesota and former Illinois governor Jim Thompson, among many others - so many that former Pennsylvania senator Rick Santorum, one of a smaller number to side with Romney, yesterday dubbed them lemmings.
There are no lemmings on the Democratic side, because no one expects either Clinton or Obama to emerge from Super Tuesday with enough delegates to claim a strong lead, let alone the nomination.
Clinton has led in national polls up until yesterday, but several new ones showed Obama inching ahead. But in the Democratic contest, winning a state doesn't count for nearly as much as under the GOP. Most states award some delegates based on the candidate's proportion of the state vote, and a greater number based on the candidate's percentage in each congressional district.
"I find it very hard to figure out how anybody's going to get much of a delegate advantage out of this," said the unaligned Democratic political consultant Bill Carrick. In Carrick's home state, California, Clinton and Obama are knotted in the polls, but even if one bursts out ahead it is likely to have only a marginal effect on the division of delegates, since candidates must win congressional districts by large margins to accumulate significantly greater numbers of delegates.
The current Democratic system was formed in two stages, the first after the 1968 campaign, when party leaders dominated the selection process to the extent that then-Vice President Hubert Humphrey could win the nomination without even running in the primaries. By 1972, the party was committed to having the majority of delegates chosen by primary or caucus, with many states instituting a proportional division of delegates, so candidates with lower name recognition could slowly build support.
At the Democrats' 1988 convention, the party moved to include an even more elaborate system of proportional representation after complaints by Rev. Jesse Jackson that he had won a substantial share of the votes in many places and had no delegates to show for it.
In years when a dominant front-runner takes hold of the contest in the early states - as John Kerry did in 2004 - the system of apportioning delegates doesn't matter: Kerry cruised to victory. But in a one-on-one race, as this year's Democratic contest has become, the intricate system for selecting delegates makes the emergence of a front-runner very difficult.
As a result, Clinton and Obama have run essentially national campaigns, making their appearances in places where they can reach the largest numbers of voters, whether or not they're likely to win the state. Clinton devoted crucial campaign time to appear on "The Late Show with David Letterman," reaching viewers across the country.
Obama began his day at the Meadowlands in New Jersey, a state Clinton has been expected to win but where he's gaining support. Standing near the stadium of the New York Giants, who beat the New England Patriots in the Super Bowl, he turned to one of his main supporters, Senator Edward M. Kennedy of Massachusetts, and quipped, "I think we should take heart, Ted, by the fact that sometimes the underdog wins."
Underdog or not, Obama stands to collect a big trove of delegates today; but so does Clinton.
"I think they're just going to have to slug it out for a while," said Carrick.
Scott Helman and Sasha Issenberg of the Globe staff contributed to this report.
