A beleaguered Jeb Bush slashed his campaign spending. Donald J. Trump lost his lead in Iowa. And a surging Ben Carson galvanized his support among social conservatives.

With Hillary Rodham Clinton emerging as the unrivaled leader in the Democratic contest, the unruly Republican presidential field suddenly seemed to lack a center of political gravity on Friday, leaving party strategists and voters to fear a long nomination fight that could end with a damaged standard-bearer facing a more unified left.

Mr. Bush cut salaries, fired consultants and laid off or reassigned many campaign workers. It was the latest sign that contenders vying for support from moderates and the party’s establishment are all but running on fumes — exhausting their cash, or the patience of their supporters, but barely moving in the polls.

Mr. Trump, for months a leading candidate, has now fallen behind in Iowa to Mr. Carson, a retired neurosurgeon, raising questions about how aggressively he will act to reverse his sagging poll numbers. And Mr. Carson, whose fund-raising has roughly kept pace with his climb in the polls, is moving to run television commercials in Iowa, New Hampshire and South Carolina, positioning himself to influence the outcomes in those states and others if he manages to continue locking down evangelical Christian voters.