Today, Occupy Wall Street protestors will be joined by members of several major unions: New York State United Teachers, Service Employees International Union, and the Transport Workers Union. They’ll stage a march at 4:30 p.m. today, and organizers estimate it will be the biggest crowd of protestors to date, perhaps drawing between 3,000–5,000 people. On other days, when the crowd has been big and mobile (the marches to Union Square and across the Brooklyn Bridge), there’s been violence and arrests; whether that will happen today remains to be seen. So far, the arrests have actually been useful to the protestors in getting publicity — the strong-arm tactics of the NYPD (who’ve been using a practice called “trap and detain” during the marches) — provided the movement with a clear villain.

But the unions’ solidarity represents another way for the protests to grow rapidly, and to potentially get a large (and organized) swath of people involved nationally, and for reasons more on-message than anger about police brutality. In addition to the groups marching today, the AFL–CIO is considering a unionwide endorsement; the Service Employees’ union is throwing a separate rally in support of Occupy Wall Street on October 12. Other unions could follow. “Mostly what we think is necessary is logistical support — sometimes technology, sometimes money, sometimes people,” SEIU’s president told Mother Jones.

Already, the unions have been involved in a supporting role. Transport Workers Union Local 100 went to court to ask for an injunction against being asked to drive arrested protestors in city buses. Plus, says Mother Jones:

SEIU 1199, in New York, recently delivered water and clothes to Occupy Wall Street protesters, as well as enough pizza to feed 200 people, spokeswoman Leah Gonzalez wrote in an email. The Amalgamated Transit Union is putting together a group of lawyers to offer legal help to Occupy Wall Street protesters if they need it, according to ATU president Larry Hanley. And ATU members are also joining the small but growing Occupy DC protests demanding an end to the flow of corporate cash in American politics.

It might be an odd-couple marriage — it’s unclear how much, say, an Anonymous activist and a city bus driver might find to talk about over a beer — but it’s one that both parties seem eager for. Unions have watched the tea party tap into a vein of populist rage that has been notably absent on the left, and Occupy Wall Street gives them something to coalesce around. The protestors, meanwhile, stand a much better chance of actually appealing to more of the so-called 99 percent with a broader coalition — not to mention counteracting the lingering perception that they’re a bunch of hippies.

Earlier: Labor Unions and Leftist Groups Prepare to Join Occupy Wall Street As Protesters Settle In

Occupy Wall Street, Powered by Big Labor [Mother Jones]