FACTBOX: Possible Democratic scenarios in Pennsylvania

(Reuters) - After a six-week break in voting in their hard-fought Democratic presidential battle, Barack Obama and Hillary Clinton square off on Tuesday in Pennsylvania -- the biggest state remaining on the party's nominating calendar.

Nine contests are left after Pennsylvania, and Clinton is struggling to gain ground on Obama in delegates to the August nominating convention and the popular vote won during the primary battle.

Clinton, a New York senator, has resisted calls to drop out of the race and allow Obama to concentrate on the match-up with Republican John McCain in November's presidential election.

Here are some Pennsylvania scenarios for the presidential nomination battle:

IF OBAMA WINS IN PENNSYLVANIA

An Obama win in Pennsylvania, where Clinton's once big lead has dwindled to single digits in most polls, would be considered a huge upset and would essentially sew up the nomination for the Illinois senator. It would kill Clinton's best hope of gaining ground in delegates and vote totals and launch a fresh wave of calls for her to drop out of the race. It also could trigger a surge of endorsements for Obama by uncommitted superdelegates, the nearly 800 party insiders who can back any candidate they want.

IF CLINTON WINS BIG

A double-digit Clinton victory would give her a boost in delegates and reinforce her argument she is the strongest candidate in big, diverse states crucial to Democratic hopes in November like Ohio, California and New Jersey. It could boost her fundraising and give her fresh momentum heading into the next two contests on May 6, in North Carolina -- where Obama has a big early lead -- and Indiana, which is considered a toss-up. Most importantly, it could send a signal to superdelegates to stay on the sidelines and let the remaining contests play out, giving Clinton more time to catch Obama or force him into mistakes like the furor over his remarks on small-town residents.

IF CLINTON WINS NARROWLY

Given her once huge lead in polls and the state's blue-collar demographics, considered ideal for her candidacy, a narrow win would increase perceptions that she cannot catch Obama. With no more big states left to vote, Clinton would have little chance left to change the dynamics of the race and build momentum for the finish. A win would likely send her on to the remaining contests that end on June 3, but pressure would continue to build on Clinton to get out of the race. Clinton aides have tried to play down expectations, and insist "a win is a win." But in Pennsylvania, a narrow win might not be enough for Clinton.

(Writing by John Whitesides in Washington; Editing by Vicki Allen)

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