Ben Carson has taken a narrow lead nationally in the Republican presidential campaign, dislodging Donald J. Trump from the top spot for the first time in months, according to a New York Times/CBS News survey released on Tuesday.

Mr. Carson, a retired neurosurgeon, is the choice of 26 percent of Republican primary voters, the poll found, while Mr. Trump now wins support from 22 percent, although the difference lies within the margin of sampling error.

The survey is the first time that Mr. Trump has not led all candidates since The Times and CBS News began measuring presidential preferences at the end of July.

No other candidate comes close to Mr. Carson and Mr. Trump. Senator Marco Rubio of Florida received 8 percent while former Gov. Jeb Bush of Florida and Carly Fiorina, the former Hewlett-Packard chief executive, are each the choice of 7 percent of Republican primary voters.

Senator Rand Paul of Kentucky, Senator Ted Cruz of Texas, former Gov. Mike Huckabee of Arkansas and Gov. John R. Kasich of Ohio each received support from 4 percent of those surveyed.

The poll represents a single waypoint in a long nominating process that has already seen considerable volatility. The third Republican debate is on Wednesday, which could further scramble the unsettled field. Such early polls have proven unreliable predictors of the eventual winner, and the first nominating contest, the Iowa caucus, is still more than three months away.

Indeed, the new survey shows that the vast majority of Republicans have not firmly made up their minds: Seven in 10 of those who expressed support for a candidate said it was too early to say for sure who they would support. Just 28 percent indicated that their minds were made up.

More than half of the Republican primary voters surveyed said they were now paying “a lot” of attention to the campaign, essentially unchanged from a CBS News survey in early October.

Over the same period, Mr. Carson has gained five percentage points, while Mr. Trump has fallen by five percentage points.

The new poll found that Mr. Trump’s supporters are firmer in their support than Mr. Carson’s. A majority of Trump supporters, 55 percent, said their minds were made up. But 80 percent of Carson backers said it was too early to say for sure that they would eventually support him.

The nationwide telephone poll was conducted Oct. 21-25 on cellphones and landlines with 575 Republican primary voters. The margin of sampling error is plus or minus six percentage points for each candidate.