Newt Gingrich got a rude welcome at a campaign stop at an Iowa grocery store on Tuesday, when a patron shook his hand and called him an obscene name.

We shook hands, I looked him in the eye and told him what I thought, Tom Sorensen, an Iowa City resident, told CNN of his insult, which involved a vulgar term for sexual intercourse and a crude reference to someone's rear end.

Gingrich represents everything that's bad about politics, Sorensen said, adding that he still hasn't decided which candidate he will support in the Jan. 3 caucuses.

Gingrich responded calmly, telling Sorensen, Luckily it's a free country, and walked off to shake another voter's hand. The incident was captured on video and posted on YouTube.

It is the latest in a stretch of bad news for Gingrich, whose poll numbers continue to decline. Just a week ago, his average was 35 percent, far above Mitt Romney's 22.3 percent. But as of Wednesday, he had fallen to 27.6 percent, just 3 percentage points ahead of Romney.

In Iowa, where he also held a double-digit lead last week, he has fallen into third place behind Romney and Ron Paul, and he was forced to admit at a recent campaign event that it had been a mistake for him to boast to ABC News earlier this month, I'm going to be the nominee. It's very hard not to look at the recent polls and think that the odds are very high I'm going to be the nominee.