I met Brian Eister in November during a demonstration at Key Bridge and was impressed by his personal commitment and insights into the leftist Occupy movement. He personifies two traits that contradict the way the protesters have been stereotyped.

Eister, 25, is no spoiled, upper-middle-class slacker on a lark. He springs from what he calls “humble” origins in Las Vegas, where he said, “Mom was a waitress, and Dad was a gambler.” He’s been active in political causes since age 17 and was a paid canvasser in the autumn in the successful reelection bid by Northern Virginia state Sen. George Barker.

And Eister has worked to help remedy the Occupy movement’s single biggest weakness: its lack of a simple, straightforward answer to the question “What do they want?”

Eister is devoting much of his energy to a campaign designed to reduce the role of big money in American government and politics. I think that cause offers the chronically unfocused Occupy movement its best chance to unify around a single issue with the potential to attract popular support.

After extended, fractious debate, the Occupy D.C. General Assembly approved a resolution Jan. 14 proposing a constitutional amendment and other measures to curb corporate involvement in elections and lawmaking.

“It came about because it’s really been the single most common thing” supported by Occupy protesters, Eister said. Although it’s only part of what’s needed, he said, “right now, with public officials basically at the mercy of campaign donors, they don’t have the leverage to create real change even if they want to.”

In the fall, Eister was sleeping in the woods to conserve his cash while he worked 90 hours a week to help reelect Barker. Now, he’s in a tent downtown in McPherson Square.

“I was able to save quite a bit of money when I was working for the Democrats,” said Eister, a slim, animated man with a small, square beard. “With really spartan living, I’ve been able to stretch that out.”

Eister first got involved in politics when he opposed the 2003 invasion of Iraq. He helped lead his high school chapter of Amnesty International and helped register Hispanic voters during John Kerry’s presidential campaign in 2004. His ultimate goal is working to improve the global ecosystem.

That kind of background is common among the people who either live at the District’s two Occupy camps, like Eister, or sleep elsewhere but regularly participate in the groups’ activities. (The second camp is at Freedom Plaza.)

When I’ve interviewed Occupy protesters over the past three months, I’ve found that many have years or even decades of experience in various liberal or left-leaning causes. They include union shop stewards. Antiwar protesters. Environmental farming advocates. Women’s health-care organizers.

Some are taking a break from causes at home, because they see Occupy as the best hope right now to build a grass-roots movement on behalf of working-class and middle-class people.

“To many people, it does seem like the most effective place to be,” Eister said.

Eister’s position interested me also because he’s committed to practical politics. Occupy needs to find signature policy goals in order to have long-term impact. Everybody knows it wants to stop exploitation of “the 99 percent” by the very rich — but how?

Talk to Occupy activists, and they want to change so many different things that it all becomes a blur. Raise taxes on the wealthy. Stop congressional regulation of the Internet. Don’t let the Pentagon treat domestic dissidents like terrorists.

Eister has chosen to focus on reducing corporations’ role in politics. He’s helped organize a couple of demonstrations — including one Saturday — to that end. He’s also working as an unpaid intern at the nonprofit group Public Citizen on a campaign to repeal “corporate personhood,” or the right of corporations to be treated as individuals under the law.

I don’t agree with all of the measures that Eister and others are supporting as part of this campaign. I’m skeptical of some of the proposed constitutional amendments. Some drafts that I’ve seen would go too far and risk weakening some important rights of media and entertainment companies, small businesses and nonprofits.

But the overall cause certainly seems right. I think there’d be widespread backing for a narrower constitutional amendment to overturn the two-year-old Supreme Court decision Citizens United v. Federal Election Commission and allow Congress to regulate corporate spending in elections.

Meanwhile, how much more time can Eister devote to unpaid activism before he needs a paycheck again?

“Another month and a half, and then it’ll be time to get a job,” he said. But he’s open to staying through what would be his one-year anniversary at McPherson: “I’ll probably be sleeping there well beyond Nov. 9, 2012, if need be.”