(Photo by Win McNamee/Getty Images)

Sen. Bernie Sanders (I-Vt.), a potential candidate for president, on Thursday expressed little faith that Hillary Clinton would be an acceptable standard-bearer in the 2016 presidential election.

"Based on her history, do I think she is going to be as bold as needs to be in addressing the major crises that we face? Probably not. I may be surprised," Sanders said in an interview with The Washington Post.

Sanders, a self-described "socialist," is considering running for president as either Democrat or an independent. Asked repeatedly about Clinton's record, he mostly declined to weigh in on specifics.

"I have no assessment," he said.

But it was clear that Sanders is not convinced Clinton, the presumed Democratic frontrunner for president, has made a forceful enough argument about how to combat income inequality, a central focus of the Vermont senator.

"Not much," responded Sanders when asked about what he has heard from Clinton on income inequality and related issues.

Pro-Clinton group Correct the Record pushed back on the former secretary of state's economic record. “Hillary Clinton has fought all her life to ensure that all Americans have the opportunity to succeed – championing equal pay for equal work, advocating for middle-class tax cuts, and pushing for a raise in the minimum wage," said spokesperson Adrienne Watson.

Sanders focused deep concern on the gap between rich and poor, an issue both Democrats and Republicans are speaking about with more frequency, and sharply criticized the billionaire industrialist Koch brothers, whose vast political network said this week it was prepared to spend nearly $1 billion in advance of the 2016 election.

"You're looking at the undermining of American democracy," said Sanders.

A Kochs spokesperson did not immediately respond to a request for comment.

As he weighs a bid, Sanders has been traveling to the early nominating states. He is headed to New Hampshire again this weekend and will return to Iowa in the coming weeks.

He said he will not run unless he thinks he "can do it well," so he does not undermine the issues he cares about.

"'Can you bring people out on the streets? Can you mobilize people? Can you tap the anger that's out there?'" said Sanders of the questions facing him as he weighs a potential presidential bid. "And the answer is, you know what, at this moment, I don't exactly know that you can."

Sanders said he plans to decide "reasonably soon" whether to run, likely before the summer.

"You can't wait indefinitely, that's for sure," he added.