More than ever, too, the party is grappling with campaign finance laws that allow candidates with wealthy private backers, such as former Gov. Jeb Bush of Florida and Senator Ted Cruz of Texas, to stay in the race even if they do poorly in early nominating contests in Iowa and New Hampshire.

But the evolving Republican landscape also suggests that the party’s changes, like squeezing primaries into a shorter period in hopes that one candidate would break through, are proving no match for a field this big and rambunctious, powered by the forces of populism and anger at Washington, and financed by wealthy benefactors.

As a result, the campaigns are preparing for a marathon delegate battle, and have begun building organizations in territories as far-flung as Guam and American Samoa. An adviser to Mr. Cruz’s campaign, Dennis Lennox, has island-hopped through the Pacific this month, discussing local issues like the airfares between Honolulu and Pago Pago, in search of a stray delegate who might support the senator. And on a conference call with donors the morning after Wednesday’s debate, Danny Diaz, the manager for Mr. Bush’s campaign, ran down its operations in states well beyond New Hampshire and Iowa, according to a participant on the call.

The prospect of a long and contentious nomination fight is only one reason for concern. The three-hour debate, at the Ronald Reagan Presidential Library near here, suggested that Republican leaders had yet to realize their hope of keeping primary contenders from moving far to the right, complicating a general election bid, as happened to Mitt Romney in 2012.