4. Former Sen. Russell Feingold of Wisconsin

Where have you gone, Russ Feingold? The former Wisconsin senator and campaign finance reform scold has virtually disappeared from the political arena. Like Clinton, he’s now serving in the State Department—as the special envoy for the African Great Lakes region and the Democratic Republic of the Congo.

Like Elizabeth Warren, Feingold would be able to rally progressives around his campaign but he could potentially have more appeal to male voters, a demographic where the party has gotten crushed in the Obama era. Unlike Clinton (and Warren), Feingold took a lone stand for same-sex marriage in 2006, when most elected Democrats opposed such legislation. He’s been a longtime critic of outside groups’ campaign spending, which has been a rallying cry for liberal Democrats in the age of the super PAC.

Feingold has always marched to the beat of his own drum, and it would be hard to see him prevailing over the better-organized Clinton. But he could persuasively assert he was ahead of the curve on the issues animating today’s Democratic Party, a powerful argument for the grassroots base. Indeed, he’d be in a situation similar to that of another reform-minded former Democratic senator, Bill Bradley, who challenged a sitting vice president and nearly won the New Hampshire primary.