FACTBOX: Scenarios in U.S. political race

(Reuters) - Democrat Barack Obama is gaining ground on rival Hillary Clinton as they head toward the Pennsylvania primary, the next showdown in their hard-fought battle 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

The next battleground is in Pennsylvania on April 22.

Opinion polls show Obama cutting Clinton's once big lead in Pennsylvania to single digits and making gains among some voting blocs that have been her most reliable backers.

Pennsylvania had been considered a lock for Clinton. A loss there would destroy her central argument in the race against Obama -- that she is the strongest Democratic candidate in the big states the party needs to win in the election battle with McCain.

After Pennsylvania, eight more contests are scheduled in May and June.

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 JoAnne Allen, editing by Todd Eastham)

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