CBS News:

On the heels of the Iowa caucuses last week, Mitt Romney now leads the field of Republican presidential candidates. Newt Gingrich is in second place, and Rick Santorum has leapt into the top tier and is now right behind Gingrich in the latest CBS News national poll.

In the race for the nomination, 19 percent of Republican primary voters support Romney, followed by Gingrich with 15 percent, and Santorum right behind him with 14 percent. This is an increase of 11 points for Santorum since last month, but a five point decline for Gingrich.

... Each of the candidates receives their support from different factions of the Republican Party. Romney's core backing comes primarily from moderates, women and non-Tea Party supporters, while Santorum's support comes from evangelicals and voters who are very conservative, the poll shows.

Gingrich seems to be losing some of his traction with Tea Party-supporters to Santorum - a social-values conservative from Pennsylvania.

While Santorum and Gingrich are now tied among Republican primary voters who support the Tea Party movement, Gingrich led this group by 16 points as recently as December. White evangelicals now seem to firmly back Santorum in this race.

However, while Santorum leads among Republican voters who identify themselves as very conservative, Gingrich is ahead among those who say they are only somewhat conservative.