It seems like a hundred polls are released every day now that the Iowa caucuses are less than a month away. Here's our guide to which ones matter and why.

Findings: President Obama now has an 8-point lead over Mitt Romney in a head-to-head contest (winning 48 percent to 40 percent), which is far better than how he fared in the same poll taken five weeks earlier, when he was behind by 1 point.
Pollster: Reuters/Ipsos
Methodology: Phone interviews taken December 8 through December 12 with 1,102 adults, among them 443 registered Republicans.
Why it matters: Obama is doing better against Republicans -- he beats Newt Gingrich, too, by 51 percent to 38 percent -- even though his approval rating has remained about the same all year, at 47 percent. And 69 percent think America is on the wrong track. On Tuesday, Obama reelection campaign manager Jim Messina told reporters that part of their strategy to win despite the bad economy was to point to how bitter the Republican primary contest was, and that the party was controlled by the Tea Party. Could it be working?
Caveat: Slate's Dave Weigel points that a NBC/ Wall Street Journal poll (discussed below) finds that a plurality of voters expect the economy to improve over the next 12 months. "All throughout 2009, voters expected the economy to bounce back," Weigel writes. "Through much of 2010, too. When it didn't, they punished Barack Obama's Democrats. I see no reason why they wouldn't do it again."

Findings:  In a two-man race, Gingrich beats Romney 59 percent to 36 percent among Republican primary voters. 
Pollster: NBC/ Wall Street Journal
Methodology: Phone interviews of 1000 registered voters, among them 200 who only use cellphones and 271 likely Republican voters.
Why it matters: One finding indicates Romney might have trouble winning back a majority of Republican voters: 53 percent of Republican voters think Romney's a moderate, while only 29 percent think he's conservative. But 28 percent think Gingrich is a moderate, while 57 percent think he's a conservative. That matters because 70 percent of Republican primary voters consider themselves conservative. 
Caveat: Republicans have only just started talking about electability, and Gingrich's lead over Romney has been declining. Gallup, for example, finds that Gingrich's lead has fallen from a high of 15 points to 9 points.

Findings: Slightly more Republicans think Gingrich can beat Obama in 2012 -- 44 percent think Gingrich can do it and 40 percent think Romney can.
Pollster: Gallup
Methodology: Phone interviews from December 8 through December 12 with 1,167 Republicans and Republican-leaning registered voters.
Why it matters: These Republican voters are wrong, at least right now. The Reuters/Ipsos poll noted above has Obama beating Gingrich by 13 points but Romney by 8. An NBC/ Wall Street Journal poll also out Wednesday found that Obama beats Gingrich among registered voters by 51 percent to 40 percent, while Obama verses Romney is much closer, at 47 percent to 45 percent. Fifty percent of registered voters said they would not vote for Gingrich, compared to 44 percent for Romney. Further, Gingrich is not very popular with seniors, the NBC poll found, winning them by 3 points, while Romney wins them by 17 points. That's interesting for three reasons: a big chunk of Gingrich's Republican supporters are seniors, 57 percent of seniors disapprove of the job Obama's doing, and old folks are hugely important in swing states like Florida.
Caveat: As noted above, it's not clear whether Romney will be able to convince voters that Gingrich can't win the general election. So far, conservative voters think Gingrich's Washington experience is a good thing.