Clinton said Iowa voters will face a choice "between someone who talks the talk, and somebody who's walked the walk," the Washington Post reported. The choice to assail Obama's character marked a major shift in campaigning tactics, the Post reported.

When asked if she plans to raise questions about Obama's character, Clinton said: "It's beginning to look a lot like that."

Obama, whose reputation for integrity is among his strongest assets as a candidate, quickly responded: "This presidential campaign isn't about attacking people for fun, it's about solving people's problems, like ending this (Iraq) war and creating a universal healthcare system.".

A Des Moines Register poll Sunday showed Obama leading Clinton by three points among Democrats likely to attend the Jan. 3 caucus. The poll's margin of error is 4.4 percentage points.