FACTBOX: Scenarios in political race

(Reuters) - Democrat Barack Obama heads into Tuesday's U.S. presidential voting having won 11 straight nominating contests, and is locked in tight races with rival Hillary Clinton in Texas and Ohio.

Republican presidential candidate John McCain, the all-but-certain nominee of his party, has big poll leads on former Arkansas Gov. Mike Huckabee in both states and could come very close to winning enough delegates to his party's nominating convention to lock up the Republican nomination.

Here are some scenarios for the presidential nomination battle:

IF OBAMA WINS TEXAS AND OHIO

Twin victories in these big states would all but sew up the nomination for Obama. His winning streak would continue, his lead in delegates to the nominating convention would grow and an air of invincibility would envelop his campaign. The so-called "superdelegates," who can vote for anyone they want, would see the trend and come over to his side. While there are still states that have yet to vote in the nominating contests, the path to victory would be extremely difficult for Clinton.

IF CLINTON WINS TEXAS AND OHIO

Her campaign will be back on track. She will have broken Obama's winning streak and the victories will add to her claim that she can win the big states -- like California, New York and New Jersey -- that are vital to the Democrats in the November election. Money that has been pouring in for the past month will continue and superdelegates will be less likely to switch. Still, because of the proportional rather than winner-take-all way Democrats allot convention delegates, Obama will still collect a large number of them even if he loses, unless Clinton wins by really big margins. Clinton would likely stay in the race but her road will still be rough.

IF CLINTON AND OBAMA SPLIT TEXAS AND OHIO

Most analysts believe Obama would come out ahead in this scenario. He will add to his delegate count and show that he can win a big state too. But Clinton will have stopped his winning streak and may want to stick around until Pennsylvania on April 22, which has similar demographics to Ohio. She will have a major rethinking of her campaign's future.

IF MCCAIN WINS BIG

A big night for McCain would push him very close to or, depending on who is doing the counting, over the magic number of 1,191 delegates needed to win the Republican nomination in September. The way Republicans have structured Tuesday's voting it will be very difficult for Huckabee to pick up more than a handful of delegates.

(Reporting by Joanne Allen, 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/)