Bernie Sanders, presidential candidate and firebrand of the left, has a lot of respect for President Obama. But he believes the president made a major mistake after running “one of the great campaigns in American history”: He left his activist supporters behind.

“The biggest mistake that Barack Obama made” was essentially to tell his supporters, “Thank you very much for electing me, I’ll take it from here,” Senator Sanders told reporters at a Monitor-hosted breakfast Thursday. “I will not make that mistake.”

Sanders, a Vermont Independent running for the Democratic nomination, has been drawing the biggest crowds of any candidate running, from either major party. The challenge he faces is to turn that interest into an organized movement.

“I do not believe that any president who’s standing up for the working class of this country can be successful without a mobilized, activist, grass-roots movement behind him or her,” Sanders said. “So I will be working hard to make sure that that mobilization exists.”

Sanders was also adamant about talking issues, not political positioning or strategy. When asked what he thought Democratic front-runner Hillary Clinton’s biggest vulnerabilities were, he balked.

“Campaigns are not baseball games,” Sanders said.

Jeb Bush has a new campaign manager? “Nobody cares!”

“This campaign is about the fact that this country faces enormously serious problems,” Sanders says.

He ticks through his list:

“Why is the middle class of this country disappearing?

“Is it moral that we have massive wealth and income inequality?

“Are the scientists right that climate change is one of the great planetary crises that we face and that we’ve got to move aggressively on it?

“Should college education be available to all regardless of their income?

“Why are we the only major country on Earth without a national health-care program guaranteeing health care for all people?"

Sanders’s solutions are controversial – a reflection of his belief in Scandinavian-style Democratic socialism. He wants “Medicare for all,” free tuition at all public universities, a $15-an-hour minimum wage, paid leave for parents of newborns, paid sick leave, and paid vacation.

Sanders acknowledges that his health-care proposal would require higher taxes, but Americans would come out ahead, he says.

“Yes, of course taxes would go up to pay for health care,” Sanders says. “But you know what would go down? Private insurance. You would not be paying private insurance. So if I said to you, ‘Well, you’re not going to pay Blue Cross $12,000 a year but you’re going to pay $10,000 more in taxes, are you going to be crying? No.”

Sanders promises to unveil a comprehensive tax plan, which his team is still working on. He said it would “end loopholes that allow corporations to stash profits in the Cayman Islands,” and include a progressive individual income tax.

Back to the question of Mrs. Clinton: “I like Hillary, I respect Hillary, I disagree with her.”

And he promises not to come out with any negative ads. He says he’s never done one, throughout his political career – from his first successful election in 1981 as mayor of Burlington, Vt., to his election to the House, then to the Senate – and doesn’t plan to start now.