Hours after Senator Barack Obama of Illinois and former Senator John Edwards of North Carolina agreed to sign a loyalty pledge put forward by party officials in Iowa, New Hampshire, Nevada and South Carolina, Senator Hillary Rodham Clinton of New York followed suit. The decision seemed to dash any hopes of Mrs. Clinton relying on a strong showing in Florida as a springboard to the nomination.

“We believe Iowa, New Hampshire, Nevada and South Carolina play a unique and special role in the nominating process,” Patti Solis Doyle, the Clinton campaign manager, said in a statement.

The pledge sought to preserve the status of traditional early-voting states and bring order to an unwieldy series of primaries that threatened to accelerate the selection process. It was devised to keep candidates from campaigning in Florida, where the primary is set for Jan. 29, and Michigan, which is trying to move its contest to Jan. 15.