The U.S. Chamber of Commerce claims credit for the ads, although a spokesman refused repeatedly to say whether it had received any funds from the Pharmaceutical Research and Manufacturers of America.
Several campaign strategists not involved in the ad campaign said no legal issues were raised by the pharmaceutical industry's involvement.
Democrats seized on the disclosure, though, to renew their charge that the program amounts to a Republican-engineered windfall for drug companies.
"There's a civics lesson here from the drug companies. They write checks to protect their GOP friends, and then they write the laws to benefit themselves, all the while doctors are writing prescriptions middle-class Americans can't afford," said Bill Burton, spokesman for the House Democratic campaign organization.
The commercials, airing in 10 states or congressional districts, generally say the local congressman or senator supports the drug program, and that hundreds of thousands of Medicare beneficiaries have saved money since its inception earlier this year.
Under the voluntary program, Medicare beneficiaries purchase prescription drug coverage from among competing plans offered by private insurance companies. Monthly premiums cover a fraction of the overall cost of the benefit, and the federal government covers most of the rest.
The insurance companies bargain with drug manufacturers over price, and the cost to consumers has been considerably lower than initially estimated. But in drafting the legislation, Republicans rejected Democratic calls to permit the government to negotiate directly in hopes of pushing down prices further.
The officials who described PhRMA's involvement said they did not know whether the industry had given the chamber money to cover the entire cost of the ads and other elements of an election-year voter mobilization effort, or merely a portion.
Ken Johnson, a senior vice president at PhRMA, issued a statement that said the organization "works with a variety of groups, including patient advocacy groups and business organizations like the U.S. Chamber of Commerce and others, to support policies that improve patient access to life-saving medicines."
In a follow-up statement, he added that as a result of the program, "millions of Medicare patients who previously had no prescription drug coverage are now benefiting from substantial discounts negotiated by Medicare drug plans."
He declined to elaborate.
Bill Miller, political director for the Chamber, did not respond to numerous requests for an interview. A spokesman, Eric Wohlschlegel, said, "The Chamber paid for the Medicare ads." But he declined repeatedly to say whether his organization had received any money from PhRMA.
In announcing the program earlier this summer, Miller described a $10 million ad campaign but made no mention of PhRMA.
The episode is reminiscent of another PhRMA-financed ad campaign, this one in May 2002.
At the time, a little-known conservative group, United Seniors Association, announced plans for a multimillion-dollar advertising effort supporting prescription drug legislation that Republicans were drafting.
A USA spokesman denied then that PhRMA had picked up the cost. But several political officials said it had, and the drug association confirmed it had made an "unrestricted educational grant" to the seniors' group.
The Chamber's current advertising effort has been marred by errors.
An ad on behalf of Republican Rep. Steve Chabot of Ohio was pulled from the air after officials realized he had voted against the legislation creating the prescription drug bill.
Commercials backing three other Republicans, Michael Fitzpatrick of Pennsylvania, Mike Sodrel of Indiana and Dave Reichert of Washington, were changed after Democrats pointed out they had not been in Congress the year the legislation passed.
At a news conference earlier in the month, Miller initially denied that either the Pennsylvania or Indiana ads had been changed until reporters showed the revised wording.