Unlike most public polls, Civis’s relied on list of registered voters that included their voting histories, allowing it to measure Mr. Trump’s support among those who regularly cast ballots in primary elections.
The survey, which was conducted on landlines Aug. 10 through Wednesday and has a margin of sampling error of plus or minus four percentage points, showed Mr. Trump’s support at 16 percent among registered voters who identified as Republicans. That tally is less than any public poll in more than a month, but still more than any other candidate. Ben Carson was at 11 percent, and Mr. Bush at 10 percent.
A poll weighted to reflect the characteristics of the adult population, like most conducted for national media organizations, would have shown Mr. Trump faring some two points better than the Civis data, which was adjusted to reflect the characteristics of registered voters who identify as Republicans. The survey included 757 Republican-leaning respondents, considerably more than other polls of the Republican presidential field.
“In reality his real support is less than what we see in the polling today,” said Masahiko Aida, lead survey scientist for Civis.
The Civis poll also hinted at a potential problem for Mr. Trump: states that allow only registered Republicans to participate in nominating contests, including Iowa and Nevada.