Even as Sen. Hillary Clinton campaigned in South Dakota, giving no signs of conceding, speculation built that an election-night rally Tuesday in New York could mark the end of her long fight for the Democratic presidential nomination.

Facing a steady stream of superdelegates flocking to Sen. Barack Obama, Sen. Clinton called on her top donors and supporters to attend her speech in Manhattan. While the Clinton campaign says such invitations have been common on primary nights, many saw it as a sign that she could withdraw from...