From Democratic Polling Firm Public Policy Polling (PPP):

There has been some major movement in the Republican Presidential race in Iowa over the last week, with what was a 9 point lead for Newt Gingrich now all the way down to a single point. Gingrich is at 22% to 21% for Paul with Mitt Romney at 16%, Michele Bachmann at 11%, Rick Perry at 9%, Rick Santorum at 8%, Jon Huntsman at 5%, and Gary Johnson at 1%.

Gingrich has dropped 5 points in the last week and he's also seen a significant decline in his favorability numbers. Last week he was at +31 (62/31) and he's now dropped 19 points to +12 (52/40). The attacks on him appear to be taking a heavy toll- his support with Tea Party voters has declined from 35% to 24%.

Paul meanwhile has seen a big increase in his popularity from +14 (52/38) to +30 (61/31).  There are a lot of parallels between Paul's strength in Iowa and Barack Obama's in 2008 - he's doing well with new voters, young voters, and non-Republican voters.

Paul's supporters are considerably more committed to him than Gingrich's are.  77% of current Paul voters say they're definitely going to vote for him, compared to only 54% for Gingrich.  Romney has much more solid support than Gingrich as well, 67% of his voters saying they're with him for the long haul. Among only voters who say their mind's totally made up, 29% support Paul to 21% for Gingrich, 18% for Romney, and 11% for Bachmann.

PPP surveyed 555 likely Republican caucus voters from December 11th to 13th. The margin of error for the survey is +/-4.2%.