Some would say "yes," but then they'd point out things that cost very little.
My mother's union ran phone banks, had a get-out-the-vote program, and sent people around to push for their candidates. A lot of the materials were produced by the union HQ, mass produced at the local level and distributed. It was really top down. With 20k union employees, they made a lot of phone calls, pushed a lot of doorbells, and handed out a lot of flyers.
Cost to the candidate or Democratic Party at any level: $0.
Cost to the union HQ: The cost of the originals of the materials sent out to locals. Perhaps $500 at most, and that's if they included the time their in-house staff spent designing them.
Cost to the local union: The cost of reproducing the originals, if they weren't just run with the union's own materials. A couple hundred dollars each.
Now, run that same program as an independant organization: You'd have to pay the organizers from the top to a fairly low level; there's be phone, postage, rent, electricity, materials preparation costs; no production costs could be buried. It would be expensive.
Since it was a union thing, union pressure was implicit. "Vote for X and support the union." I seriously couldn't ever imagine any church or school or organization I've been in trying the same stunt: "Vote for X and support the church" or "Vote for X and support the Scouts" or even "Vote for X and support the American Association of Teachers of Slavic and East European Languages."
Union's financial clout: Not so much. But you'd need tens if not hundreds times their costs to equal their actual clout. This, of course, depends on the union and how it's perceived by the citizenry and by its own members. Where I live, the teachers' unions have pretty much zero clout. In other states, the NEA or even the less active ATF has significantly more clout.
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