But it is likely that Mrs. Clinton’s campaign, which is especially sensitive to criticism of her connections to Wall Street, will view the ad as negative.

The moment for Mr. Sanders is a significant one. If he was to prevail in Iowa and then go on to win in New Hampshire, where he is favored, he could generate momentum and financial support to present a serious challenge to Mrs. Clinton. A loss in Iowa, however, would probably diminish his standing, and his campaign could quickly lose steam.

In an exclusive interview on board his campaign plane on Tuesday, Mr. Sanders seemed to be wrestling out loud with the conflict between his desire to preserve his reputation for positive campaigning and his eagerness to defeat Mrs. Clinton. He rejected any suggestion that his candidacy was only symbolic.

“We want to win,” Mr. Sanders said. “We think we have a good chance to win. We think Iowa has a historical role to play in making it clear that the American people want to move this country in a very different direction, away from establishment politics and establishment economics. And that’s what we hope will happen.”

In the interview on the plane, a leather-upholstered Gulfstream jet, during which he snacked on cheese wedges and kiwis, Mr. Sanders repeatedly described his campaign as positive. Nonetheless, he questioned Mrs. Clinton’s acceptance of more than $600,000 from Goldman Sachs for giving three speeches.

“I am surprised that Hillary Clinton does not understand why so many people have strong concerns about her receiving many, many hundreds of thousands of dollars from Goldman Sachs and from many other financial interests,” Mr. Sanders said. “And I think people don’t understand why she doesn’t see the concerns that Americans have when you receive millions of dollars in speaking fees from Wall Street, the most powerful entity in America. You know, people can draw their own conclusions, but that is just simply a fact.”