WASHINGTON — The television ad sponsored by the advocacy arm of the National Federation of Independent Business featured a small-business owner in Arkansas, frustrated at what he said were the higher bills he had seen since the Obama administration’s health care plan went into effect — and pointing blame at Senator Mark Pryor, a Democrat considered one of the most vulnerable incumbents facing re-election this fall.

But the largest chunk of the money donated to the nonprofit group’s advocacy effort came not from small-business owners, but rather from health insurance companies trying to repeal a health care tax, the most recently available federal tax records show.

The largely hidden role of the for-profit health insurers highlights the increasingly confusing world of campaign finance, as nonprofit groups such as the National Federation of Independent Business and its Voice of Free Enterprise program can keep their donor lists secret and then present their carefully fashioned message, financed in large part by big business, as if it is coming from, perhaps, a more sympathetic voice.

“If people who see this ad have no idea who is actually bankrolling it, they are in effect being misinformed,” said Sheila Krumholz, the executive director of the Center for Responsive Politics, a nonprofit that tracks the influence of money in politics.