Scott Walker, who had emerged as an early strong contender for the nomination, lost 21 percent of his initial support to Mr. Trump, and 32 percent of them said they would come back.

Whose voters were the most loyal? Almost 70 percent of John Kasich’s supporters from the spring stuck with him through the summer. Rand Paul, Mr. Cruz and Mr. Bush also held on to 40 to 50 percent of their early fans.

If Mr. Trump left the race tomorrow, this survey finds that the biggest share of his supporters would go to Ben Carson, who would collect 18 percent of Mr. Trump’s current supporters. Carly Fiorina is next in line, followed by Mr. Cruz and Mr. Rubio, each garnering around 12 percent of Mr. Trump’s voters.

All of this shifting by voters among the candidates underscores a clear pattern in primaries: They are volatile. Primary campaigns are a lot more dynamic than general election campaigns, mainly because of the lack of party identification as an organizing force and the importance of news media coverage in shaping people’s choices.