An estimated 200 social justice protesters demonstrated in Freedom Plaza in Washington, D.C., Thursday, expressing solidarity with the “Occupy Wall Street” movement that has spread to more than 150 US cites.

The protesters, part of a larger group that had gathered earlier near Capitol Hill for a long-planned antiwar rally, beat drums, carried signs that said “Tax the Rich,” and rang cowbells.

The smaller group’s modest, if opportunistic, appearance in Freedom Plaza showcased a larger phenomenon challenging the Occupy Wall Street movement, which began Sept. 17 in New York.

As expressions of solidarity with Occupy multiply – labor unions marched in New York Wednesday while student and labor groups joined in Boston – the question is being asked: Can the protest movement carry the newcomers, or will it sink under their weight?

Political scientists, sociologists and historians – as well as public relations specialists – are coming forward to offer their views and comments about the pluses and minuses of merging interests with other groups, some traditional, others not.

Partly because the movement has coalesced so quickly and captured growing media coverage, it faces both uncommon promise and peril as it tries to turn the corner and sustain itself for the longer term. The next two weeks are crucial in deciding its identity and structure, say a host of experts who study grass-roots political movements.

“The entropy that is Occupy Wall Street threatens to either destroy the group or propel it to new heights,” says Michael Robinson, senior vice president for Levick Strategic Communications, an international public relations firm. “They need to be really careful about becoming too schizophrenic and having too many personalities. They risk getting diffused if they expand themselves too much.”

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Pros and cons of union involvement

The entry of established groups could benefit the movement by providing it with focus and organization, says Jack Pitney, a political scientist at Claremont McKenna College in California. “But these groups could also jeopardize the protests by making them look like just another partisan political tactic. The involvement of labor unions is especially problematic. Public approval of unions is near historical lows,” he says.

Pitney, Robinson, and others point out that the antiwar movement of the 1960s had a singular goal – getting the US out of Vietnam – and even the Arab Spring countries zeroed in on very specific goals such as “Get Mubarak Out” rather than a laundry list of complaints from water rights to the economy.

The Occupy Wall Street movement is circulating a working draft of very broad principles – “engaging in direct and transparent participatory democracy,” for example, and “the belief that education is a human right” – and different cities are coming up with specific goals in coming days.

Standing outside his tent at Los Angeles City Hall Wednesday, activist Joe Briones said breadth is precisely the appeal of this movement to him.

“In the '60s, you had the civil rights movement and Vietnam with specific goals, but once those goals were met, the activists had nowhere to go,” he says. “We are building this for the long term.”

If that’s true, all the more danger and promise at the moment, say others. The more you tighten goals with specifics, the more people get alienated.

“I think this is going to get dragged down by all these splinter groups,” says David Johnson, CEO of Strategic Vision LLC, a public affairs polling and branding consulting firm. “Their great appeal at the beginning was that they had people from left, right, and middle involved. But now we have the president mentioning them at a major press conference, and others like [former Wisconsin Democratic senator] Russ Feingold, and the movement is beginning to look like a liberal response to the tea party, rather than a genuine apolitical move.”

Historical perspectives

Turning to history for lessons, Mr. Johnson says America’s populist movement was co-opted and overtaken by the Democrats in the late 1880s, thus losing its message, and the progressive movement was similarly overtaken by Franklin Roosevelt’s New Deal.

Dr. Ben Agger, director of the Sociology Department’s Center for Theory at the University of Texas, Arlington, fires off an even longer string of comparisons in an e-mail, but predicts the movement will endure.

“A leaderless street action joining workers, unions, and students in massive resistance to capitalism, occurring in and beyond a major city?! That sounds like the Paris Commune of 1871, the May Movement in Paris in 1968, the Weatherman-led Days of Rage in Chicago 1969, the 1999 Seattle WTO protest,” he says. “It also sounds like the so-called Arab Spring (still occurring) that is using a combination of street actions and the Internet to overthrow dictatorial power in Middle Eastern countries such as Egypt.”

Professor Agger says Occupy Wall Street is occurring for the usual reasons laid out by Karl Marx: class struggle as a response to the failures of capitalism.

“Marx predicted the overthrow of capitalism from below, by workers. He perhaps underestimated the capacity of the welfare state (FDR) and the culture industry (Hollywood, the Internet, shopping) to bleed off revolutionary discontent. But have economic crisis and class struggle disappeared? Occupy Wall St., the latest street action against capitalism, suggests not.”

Activity behind the scenes?

Others say the movement hasn’t been as “spontaneous” as it has appeared from the beginning, and so the question of outside involvement is somewhat moot.

“While it is (intentionally) unclear for how long or how much unions have been involved in planning, supporting and funding the current Wall Street protests, there is little doubt that their fingerprints are on it either directly or through front organizations,” says Phillip Wilson, president of the Labor Relations Institute. “What we know for certain is unions will now be openly providing the organizing and financial support a large sustained protest needs.”

Some say it will be how Election 2012 plays out that will determine the movement’s staying power.

“What they may accomplish is moving Obama more toward his liberal base, which is where he has been tacking since just after Labor Day,” says Villanova University political scientist Lara Brown, author of “Jockeying for the American Presidency.” “And if they do that, then yes, you may see more Democrats, liberals, progressives – including union members and minorities – becoming reengaged and more enthusiastic about a second Obama term.”

In short, she says, “it is not the movement, per se, but whether or not the president joins with them or not, which will determine whether they have both a lasting and meaningful effect on the 2012 general election.”

Nina Eliasoph, a sociologist at the University of Southern California who studies grass-roots movements, says no matter what happens, Occupy Wall Street is probably here to stay.

“We are in a moment where there is a bigger gap between rich and poor than anytime since 1929, and so people are forced into political engagement,” says Ms. Eliasoph. “The organizations in these coalitions have been around for decades, so even if these specific street occupations fizzle, people and leadership will still be around from the feeder organizations.”

• Staff writer Gloria Goodale contributed to this article.