Robert Reich has been friends with Hillary Clinton for half a century. He marched with her in civil rights demonstrations when they were in college. Reich’s fellow Rhodes Scholar, Bill Clinton, called him “one of my best friends” and appointed him to what Reich calls “my dream job,” secretary of labor.

So given all that personal and professional history with the Clintons, it is telling that the UC Berkeley professor has endorsed Bernie Sanders for president. Being besties, apparently, has its limits.

And that limit is the question of how the nation should deal with the dangerously wide income inequality gap blamed for killing the middle class.

For the past three decades, Reich has been a leader in sounding the alarm that the wealthiest Americans have a choke hold on the nation’s political and financial systems. (Check out Reich’s engaging documentary “Inequality for All. ” )

Few wanted to listen when he was in the Cabinet because the economy had improved during Clinton’s term. While Bill Clinton was sympathetic to the issue, Reich said his administration never addressed the economic underpinnings that widened income inequality. It was one of the reasons he left the administration after Clinton’s first term.

After his departure, the White House helped kill Glass-Steagall, a 1933 law that had required commercial banks to separate their banking and investment activities. Many see its demise as a starting point for the 2008 financial meltdown and Great Recession.

Yes, Reich said, Hillary Clinton was her husband’s top (unofficial) adviser. But no, Reich didn’t know how much she had to do with his economic policies or with killing Glass-Steagall.

At this point, he said, it’s more important that candidate Hillary Clinton has said she doesn’t want to bring it back — and Sanders does.

“Given everything that I’ve stood for over the years, it would have been bizarre for me not to support Bernie Sanders,” Reich said. “Bernie Sanders exemplifies everything that I’ve been fighting for.”

Reich, whose opinion column appears in The Chronicle’s Sunday Insight section, says his endorsement of Sanders is not about Clinton. At least, not personally.

“I have enormous respect for Hillary Clinton. If she wins (the Democratic nomination), I would work my heart out for her,” Reich told me. And he wasn’t saying it in that smarmy way where Beltway types call someone “a great American” as a backhanded insult.

“She’s the best-qualified person in the field to be president in our current system, but that’s precisely the problem,” Reich said. “The system we have doesn’t work now. It’s corrupted by big money.”

He said Sanders has “the passion and the authenticity” to take on Wall Street and the elites that control the system.

Does Clinton?

“I frankly don’t know,” Reich said. “And that’s part of her problem with voters. If she is the nominee, the only way to get that Sanders type of enthusiasm is for her to do what Bernie has done — and that’s be part of a movement.”

“The reason Bernie is popular is not because of Bernie,” Reich said. “It’s not because of Bernie’s charisma or good looks or charm or sense of humor. It’s because he is channeling a movement that has grown over the years and is gaining force in America.”

Clinton clearly has an enthusiasm problem. Look no further than the latest California Field Poll, which came out Friday. A little over a year ago, Clinton led Sanders by 63 points among the likely Democratic primary voters surveyed; now she leads him by six. In January, she held an 18-point lead among Latinos; now it’s seven. While 61 percent of Clinton supporters think favorably of Sanders, only half of his backers think positively about Clinton.

More ominous is that more California registered voters of all parties view Clinton unfavorably (48 percent) than favorably (47 percent.) That’s a big red flag in deep-blue California, where Clinton has raised upward of $24 million and defeated then-Sen. Barack Obama in the state’s 2008 primary.

A couple of years ago, Reich offered Clinton several ideas on how to harness the energy of the income-inequality issue. Some she has backed (independent analysts say her tax plan would soak the 1 percent of the richest Americans for an average of $78,000 more); some she’s half-embraced (she supported raising New York’s minimum wage to $15 an hour, but not the federal wage); and some she’s blown off (reinstating Glass-Steagall).

But Reich hasn’t spoken to Hillary Clinton in at least a year. He and Bill Clinton, his one-time bestie, “are no longer in touch. We don’t really travel in the same circles.” But he still considers both to be friends.

Does he think they hold a grudge against him for endorsing Sanders? Or, for that matter, for endorsing Obama for entirely different reasons back in 2008? Or for calling them out for not being up to taking on the top issue facing the country?

“I have no idea,” Reich said. “They’re politicians. If they hold grudges, they hold them for political reasons.”

Joe Garofoli is The San Francisco Chronicle’s senior political writer. Email: jgarofoli@sfchronicle.com Twitter: @joegarofoli

Editor’s note

Each Monday, senior political writer Joe Garofoli will break down national, state and local political issues of the day in this column.