Sanders: We're taking on the 'Democratic establishment'

The surest sign that Bernie Sanders is moving up in the polls -- he's being criticized by supporters of Hillary Clinton and other Democrats.

"It's very clear to say that Secretary Clinton is the candidate of most of the members of Congress, is the candidate of the Democratic establishment," Sanders said Sunday on CNN's State of the Union.

The Vermont senator added that "ordinary people, working people, elderly people are moving in our direction because they do want a candidate to take on the establishment."

Sanders responded to comments by Sen. Claire McCaskill, D-Mo., a Clinton backer who challenged Sanders' Democratic credentials during a recent interview on MSNBC.

"I very rarely read in any coverage of Bernie that he's a socialist," McCaskill said. "I think everybody wants a fight and I think they are not really giving the same scrutiny to Bernie Sanders that they're giving certainly to Hillary Clinton and the other candidates."

She added, "I think Bernie is too liberal to gather enough votes in this country to become president."

Sanders, who has described himself as a "Democratic socialist," is also drawing huge crowds at his campaign rallies, and is generally in second place behind Clinton in early polls.

In his CNN interview, Sanders said people are responding to his issues, including the growing wealth gap, the corrupting influence of money on politics and "the fact that we have a media today that looks at politics as if it were a soap opera or a baseball game, rather than allowing us to focus on the real issues facing the American people."