Sen. Barack Obama of Illinois hopes to get some delegates in the New York and New Jersey Democratic primaries Feb. 5, even though he probably won't win either one.

And former New York Mayor Rudy Giuliani's entire strategy hinges on winning several key Republican primaries, including New York and New Jersey, because he would get all the delegates.

It's all a matter of rules.

As those situations demonstrate, the rules influence how presidential-nomination politics is played, and they could take on added and unaccustomed significance this year if either race stays close past Super Tuesday on Feb. 5.

The rules could influence how long the contests last and even who prevails, both in the increasingly bitter Democratic struggle and the more civil Republican one.

They vary widely from one state to another, and between parties in the same state, on such matters as how convention delegates are allocated - proportionately or winner-take-all - and whether independents can vote.

"Both parties have approaches that are keeping with their general philosophy," said Robert DiClerico, a political scientist at West Virginia University. "Republicans think states should be free to do what they want. Democrats see it as a matter of what is equitable."

Even with the rules differences, the contests are similar in this regard: Mathematically speaking, it is close to impossible for any candidate, Republican or Democrat, to wrap up the nomination on Super Tuesday Feb. 5, when 22 states vote. In political reality though, that day could prove decisive.

For the Democrats, national party rules require that all delegates be allocated proportionately.

The rules make it possible for Obama to get delegates in states that New York Sen. Hillary Rodham Clinton figures to win - "You can aggregate delegates even in someone's backyard," said David Plouffe, Obama's campaign manager - and hard for any candidate to quickly accumulate the 2,000-plus delegates needed for nomination.

That will be particularly true if former North Carolina Sen. John Edwards can stay in the race and do well enough to win delegates.

Proportional allocation also explains how Clinton could defeat Obama in Nevada on Saturday but gain little or no advantage in delegates.

On the Republican side, the national party has no rules on the subject, freeing state parties to distribute delegates as they choose.

Florida, site of the next Republican showdown, on Tuesday, has a winner-take-all primary. The same is true for many of the Republican contests Feb. 5, including Delaware and New Jersey. Twelve more states scheduled later in the season are winner-take-all.

Giuliani's strategy depends on winner-take-all events, starting with Florida. Were he to win enough of them, he figures, he would have the delegate lead Feb. 6, despite his poor showings to date.

With so many winner-take-all states set for Super Tuesday, any GOP candidate who runs the table, most likely Arizona Sen. John McCain or former Massachusetts Gov. Mitt Romney, could build up a formidable delegate lead.

The winner-take-all option discourages Republican candidates from campaigning in states they don't think they can win; for that reason, former Arkansas Gov. Mike Huckabee pondered giving up in Florida, although he decided not to.

Another set of rules that influences outcomes - and this is a matter of state law - concerns who participates in a primary or caucus.

The issue is whether independents can vote, and how easy it is for them to do so. And it matters.

McCain might not have won either of his two big victories, in New Hampshire and South Carolina, if those primaries had been limited to Republicans. Obama did very well among independents in his one win to date, the Democratic caucuses in Iowa.

Primaries and caucuses can be closed, open, or something in between.

Closed events are those in which only individuals registered in advance as members of a party may vote. One of those is the Pennsylvania primary April 22.

Open primaries and caucuses take place in states that have no partisan registration, such as South Carolina.

Anyone can vote in South Carolina's Democratic primary Saturday so long as he or she did not vote in the Republican primary last Saturday.

There are few open primaries Feb. 5. That could help Clinton; she has been stronger than Obama thus far with self-identified Democrats.

Then there are "semi-open" events. These occur in places that have partisan registration and allow independents to vote in the primary of their choosing.

Rules on how this works vary from state to state.

In New Jersey, independents may vote in a primary, but they become members of that party by doing so.

Another rules-related matter that could come into play later in the spring if either race is still unresolved - and could be the source of contention - concerns the states that have violated party rules and have been penalized for it.

On the Democratic side, for scheduling their primaries earlier than the party wanted, two states had all their delegates stripped away.

On the Republican side, four states lost half their delegates for the same reason.

Democratic and Republican leaders in those states say they are confident they will get their delegates back in time for the national conventions.

Their wishes likely will be fulfilled if their presidential nominee is in place by then. But credentials fights could be in the offing if those delegates would help determine who gets the nomination.

Should one candidate in either party start winning everything in sight, none of these intricacies will make any difference. Indeed, Clinton, noting the importance of Michigan and Florida in the general election, said yesterday that she would support seating both states' delegations at the Democratic National Convention in Denver.

Otherwise, you'll hear a lot in the days to come about the rules.

Updates from South Carolina, Florida, plus more coverage, video and photos at