Four presidential candidates are new clustered between 17 and 20 percent support among likely Iowa Republican caucus-goers, according to Selzer and Co’s latest poll of Iowa Republicans, conducted for Bloomberg News.

John McCormick reported the toplines for Bloomberg: former Godfather’s Pizza CEO Herman Cain has 20 percent support, Representative Ron Paul has 19 percent, former Massachusetts Governor Mitt Romney has 18 percent and former House Speaker Newt Gingrich has 17 percent.

The poll reflects the race’s fluidity, with 60 percent of respondents saying they still could be persuaded to back someone other than their top choice, and 10 percent undecided. Paul’s support is more solidified than his rivals, while Cain’s is softer. All of the major contenders have issue challenges to address.

“In Iowa, it’s long been a two-person race between Romney and someone else,” said J. Ann Selzer, president of Selzer & Co., which conducted the poll for Bloomberg. “It is now a four- person race between Romney and three someone elses.”

Texas Governor Rick Perry registered 7 percent support in the new poll, while 5 percent of respondents back Representative Michele Bachmann, 3 percent back former U.S. Senator Rick Santorum and 1 percent favor former Utah Governor Jon Huntsman.

The new poll was conducted between November 12 and 14 and has a margin of error of plus or minus 4.4 percent. I will update this post if more details about the sample, likely voter screen or questionnaire become available.

Romney’s solid block of support around the low 20s in most Iowa polls is starting to look like a ceiling as well as a floor in Iowa. I would have thought that by now, more Republicans would be gravitating toward him as the alternatives appear less viable for a general election campaign against President Barack Obama.

Paul’s never been so close to the 20 percent mark in any Iowa poll I can remember. He’s spent the most money on television advertising in Iowa, but that hasn’t moved his numbers much in the past. Having the most committed supporters may be an advantage when it comes to getting people out of their houses on a cold January evening.

Clearly I was too quick to declare Gingrich irrelevant in Iowa. Despite relying on volunteers and not spending as much time in the state as some other candidates, Gingrich has become a real contender again, thanks to strong debate performances and the overall weakness of the GOP field.

Selzer’s last poll of Iowa Republicans for the Des Moines Register, conducted October 23 through 26, found Cain slightly ahead with 23 percent support, followed by Romney (22 percent). Paul was in third place at 12 percent, and everyone else was below 10 percent in that poll.

Any comments about the Republican presidential race are welcome in this thread.