FACTBOX: Scenarios in U.S. political race

(Reuters) - Democrat Barack Obama beat rival Hillary Clinton in Mississippi on Tuesday, adding to his weekend win in Wyoming and giving him new momentum as they head into the next critical showdown in Pennsylvania in six weeks.

Republican candidate John McCain, who also won in Mississippi, had already clinched his party's presidential nomination for the November election.

Here are some scenarios for the presidential nomination battle:

DEMOCRATS

The next battleground is in Pennsylvania on April 22. Clinton has a double-digit lead in public opinion polls in the state, which has many similarities to Ohio, where her victory last week kept her presidential hopes alive. After Pennsylvania, eight more contests are scheduled in May and June.

With his two further wins, Obama has built on his lead in pledged delegates to the August national convention that will pick the next nominee. Clinton will have a tough time cutting into that lead and may have to rely on the "superdelegates" who can vote for anyone they want.

REPUBLICANS

With no serious Republican to challenge him, McCain has begun a national campaign and is trying to set the agenda for a autumn race against either Obama or Clinton. He has scheduled a foreign trip next week to Europe and the Middle East.

(Writing by Lori Santos, editing by Patricia Zengerle)

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