Updated 5:35 p.m. ET

DES MOINES -- A new poll out today shows Mitt Romney leading in Iowa as Newt Gingrich fades and Rick Santorum gains ground six days before the first-in-the-nation presidential caucuses.

Romney is the choice of 25% of likely Iowa GOP caucusgoers in the CNN/Time/ORC International poll, followed by Ron Paul with 22%. The 3-point difference is within the poll's margin of error.

The surprise is the upward movement by Santorum, a former Pennsylvania senator who has won the backing of social and Christian conservatives who play a key role in the Iowa caucuses. He has 16% support from likely GOP caucusgoers -- up 11 percentage points since the beginning of the month in the CNN/Time/ORC International survey.

Gingrich, who started climbing in Iowa polls in November as Herman Cain faltered, has 14% support and falls behind Santorum in the latest survey. The former House speaker's lead evaporated amid an onslaught of negative advertising from his rivals, especially Romney and Paul, and their allies.

Time magazine's Adam Sorensen says this about Gingrich in his analysis of the poll:

Some of his voters have scattered, providing small bumps to Romney and Paul as well as Minnesota Rep. Michele Bachmann and Texas Governor Rick Perry. But the biggest beneficiary of Gingrich's collapse appears to be former Pennsylvania Senator Rick Santorum.

Santorum's surge helps make the race in Iowa a volatile one. More than four in 10 likely caucusgoers, or 43%, said in the latest survey that they could change their mind about their GOP presidential choice before Jan. 3. Time says the outcome "remains incredibly unpredictable."

Keating Holland, CNN's polling director, says most of Santorum's gains are among likely caucus participants who describe themselves as born-again Christians or evangelicals. Santorum outpaces Paul, Romney and Gingrich in this crucial bloc, Holland is quoted as saying in CNN's Political Ticker blog.