FACTBOX: Scenarios in U.S. political race

(Reuters) - Hillary Clinton and Barack Obama face off on Tuesday in primaries in Indiana and North Carolina, the latest battlegrounds in their hard-fought duel for the Democratic presidential nomination.

Republican candidate John McCain has already clinched his party's presidential nomination for the November election.

Here are some scenarios for the presidential nomination battle:

DEMOCRATS

Obama still leads Clinton, a New York senator, in pledged delegates to the August nominating convention and in votes won during the four-month primary battle.

Obama, an Illinois senator, has seen a once double-digit lead in North Carolina shrink to single digits in most polls after a rough campaign stretch that included a loss to Clinton in Pennsylvania and the resurfacing of his controversial former pastor.

The race in Indiana is tighter, with most polls giving Clinton a slight edge. Independents are able to vote in Indiana's Democratic contest, which could help Obama since he has attracted those voters in the past.

Both candidates are hoping for a sweep in the two states, which have a combined 187 delegates at stake.

If Obama wins both, it would likely spark another flood of calls for Clinton to step aside and allow Obama to focus on the race with McCain. It also would dash Clinton's hopes of catching him in delegates or votes.

Clinton victories in both states could fan the doubts about Obama's electability and persuade some superdelegates -- party insiders who are free to back any candidate at the nominating convention -- to move toward her.

After those two states, just six more contests will remain before the Democratic race concludes on June 3. Clinton is favored in Kentucky, West Virginia and Puerto Rico, while Obama is favored in Oregon, Montana and South Dakota.

Neither candidate will win enough delegates in the state contests to clinch the nomination, leaving the decision at that point to the party's nearly 800 superdelegates.

Both Clinton and Obama want to show them they are the candidate with the best chance to beat McCain in November.

Many Democratic strategists expect a wave of superdelegates to move toward the leader after the final contest on June 3, essentially deciding the race before it reaches the convention in Denver.

REPUBLICANS

John McCain, who has clinched his party's presidential nomination, has begun a national campaign and is trying to set the agenda for a autumn race against either Obama or Clinton.

(Writing by John Whitesides, editing by David Wiessler)

(For more about the U.S. political campaign, visit Reuters "Tales from the Trail: 2008" online at http:blogs.reuters.com/trail08/)