Bernie Sanders has closed the gap on Hillary Clinton in national polling and in fundraising, but he's clinging to the hope that he can win without going negative on her.

At least for now.

In what is the most compelling evidence yet that Sanders is serious about winning the Democratic nomination, and the presidency, he told former Obama campaign strategist David Axelrod that he hasn't ruled out running negative ads for the first time in his career.

"It’s my hope that I will never run a negative ad," Sanders said this week in an interview with Axelrod, who is now in charge of the University of Chicago's Institute of Politics. "I never have after all these years. But I — we’ll see."

While Axelrod didn't target Clinton specifically with his question — asking if Sanders could get through the entire 2016 campaign without negative ads — Sanders is already drawing contrasts with Clinton on the stump.

Running negative ads would be a tougher call for Sanders, who raised $26 million to Clinton's $28 million — not counting her Super PAC funds — in the reporting period that ended September 30. On the one hand, running clean campaigns has been a hallmark of his political career and a point he emphasizes at campaign stops. On the other, as I wrote after watching Sanders on a two-day swing through Iowa this past weekend, he may not have done enough yet to drive voters away from Clinton:

He may have to sharpen contrasts with Clinton — to turn more Democrats against her — to make the case that he'd be a better nominee and president. Sanders will have to decide whether it's worth risking his brand to do that.

"I think she has a better chance of winning, and I really want to see a Democrat as president," Gail Klearman, who plans to caucus for Clinton, told me after a Sanders event in Waukee on Sunday. "I agree with him on more issues than I do with Hillary. But not by a whole lot."

It's that distance Sanders would try to widen if he hit the airwaves with attacks on Clinton. Either way, the fact that Sanders is entertaining the possibility of running negative ads is an indication that he sees himself as an increasingly serious contender for the presidency.