As Hillary Rodham Clinton moves steadily toward what Democrats now see as an inevitable campaign for president in 2016, Sen. Bernie Sanders (I-Vt.) is readying a dissent from the left.

Sanders has been traveling the country to explore the possibility of running his own campaign in 2016. He has been to neighboring New Hampshire and to South Carolina and is thinking about a western trip. On Saturday night, he was scheduled to give the keynote address at the Clinton County, Iowa, Democratic fundraising dinner.

He won’t commit yet to running and will be written off by many if he decides to do so. Sanders says he doesn’t know whether Clinton will run or not but, like everyone, can see the direction of things this spring. He already has plenty of questions about whether the former senator and secretary of state is what he believes the times demand.

“If she does run, will she be as strong as the times require in taking on the billionaire class that has so much power? I’m not sure that she will be,” Sanders said during an interview in his Senate office. “Will she be as strong as needs be to address the crisis of climate change? I am not sure that she will be. Will she be as strong as needs be to take on the power of Wall Street? I’m not sure she will be.”

It has been widely assumed that if Clinton runs, someone who speaks for the party’s restive progressive wing could mount some kind of challenge — symbolic, gadfly or otherwise — to force a more robust discussion about economic issues and the power of Wall Street and corporate interests. Sen. Elizabeth Warren (D-Mass.) is a popular choice among many on the left to lead that effort, but she has so far demurred. Sanders is not so reticent.

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He appears slightly uneasy presenting himself purely as the anti-Hillary. “I like Hillary and have known her for many years,” he said somewhat defensively after he was reminded that he recently told Jay Newton-Small of Time magazine that he would be a better president.

“What I do know about Hillary Clinton is that she has been a very strong advocate of women’s rights,” he continued. “I respect that. She and I have worked together on some issues regarding children. She’s been a strong advocate of children. I have a lot of respect for Hillary Clinton.”

But it’s clear his respect has limits when it comes to the core issues that long have animated his politics. “These are extraordinary times, which require a boldness and an aggressiveness that I’m not sure her past history suggests is there,” Sanders said. “I am not sure that she has been — ” He paused and caught himself. “Well, that’s all. I’m going to leave it at that.”

Sanders is the longest-serving independent member of Congress in history. He was elected to the House in 1990 and to the Senate in 2006. Before going to Washington, he was the socialist mayor of Burlington. He still calls himself a socialist. Asked if he regards President Obama, who has been called a socialist by critics on the right, as a true progressive, he said, “No, he is a moderate Democrat.”

Sanders was born in New York in September 1941 and still carries the Brooklyn accent of his youth. The opening lines of his first Senate ad in 2006 sum up his political philosophy: “My father came to this country from Poland at age 17. Worked every day of his life. Ever since I was a little kid, I just don’t like to see ordinary people, people without a lot of wealth, put down and pushed around.”

In the Senate, his record of effectiveness is considered mixed. He is a fixture on prime-time MSNBC and has developed a substantial grass-roots following around the country on the left, aided by an eight-hour floor speech in 2010 in which he railed against the privileged interests.

The nation’s No. 1 problem, he said, is the “collapse of the middle class and income and wealth inequality.” He said the amount of wealth going to the top 1 percent is “obscene.” Close behind this economic imbalance on Sanders’s list of concerns is the threat of global climate change.

Sanders advocates a traditional liberal platform to address income inequality: raising the minimum wage (and $10.10 an hour is, to him, a floor), a massive federal jobs program to rebuild the country’s infrastructure, more exacting corporate income taxation, a higher estate tax. He would take the proceeds of additional tax revenues and redistribute them to help working families pay for child care or college tuitions. To combat climate change, he favors a tax on carbon and programs to fund energy-efficiency measures and alternative sources of energy.

The third big problem, Sanders said, is the “unprecedented way” some of the country’s wealthiest people — he always mentions the Koch brothers, David and Charles, and casino owner Sheldon Adelson — are using their wealth to pursue their political agendas. He rails against the Supreme Court decision in Citizens United v. FEC for having spawned the era of super PACs.

“I think it is obscene that billionaires — Democrat, Republican, independent, whatever — play a significant role in our political process,” he said. “That is not what democracy is about.” He would deny all of them an outsize voice by instituting public financing of campaigns.

But Sanders said he believed that it is “a false equivalence” to compare the influence of billionaires on the right and left. “Some people say, well you’ve got several billionaires [on the left],” he said. “You’ve got [George] Soros, you’ve got Tom Steyer spending a lot of money. But the truth is it’s not equivalent. The Koch brothers will spend as much as it takes.”

He sees those conservative billionaires as more effective. Asked which movement — the tea party, which he opposes, or Occupy Wall Street, which he supported — has been more potent politically, he said, “I don’t think there’s any question. The tea party has been very successful.”

Sanders was quick to add that the tea party movement would not be so successful without funding from people like the Kochs. “I disagree with everything the Koch brothers stand for,” he said, “but I will sit here and tell you they have been extraordinarily successful.”

Sanders said he will decide sometime later whether to launch a presidential campaign. Already he envisions constraints. He will not be a spoiler by running an independent candidacy that would drain votes from the Democratic nominee. “If I chose to run . . . and if my candidacy were to take off, I do not want to be in a position to elect some right-wing Republican,” he said.

Some people believe Sanders has little to lose in such a campaign. Perhaps. When asked if he is concerned about the prospect of Clinton claiming the nomination without being challenged, he said, “It’s not just acclamation for Hillary, it’s that there are millions and millions of people out there — I see them every day — who are hurting, who are struggling. . . . Somebody has got to defend those people.”