Senator Hillary Rodham Clinton and her advisers increasingly believe that, after a series of losses, she has been boxed into a must-win position in the Ohio and Texas primaries on March 4, and she has begun reassuring anxious donors and superdelegates that the nomination is not slipping away from her, aides said on Monday. Mrs. Clinton held a buck-up-the-troops conference call on Monday with donors, superdelegates and other supporters; several said afterward that she had sounded tired and a little down, but determined about Ohio and Texas.
