Conservative groups are gearing up to spend millions of dollars over the next three months on ads attacking President Obama’s health-care law and Democrats who support it, but in many cases voters will have no way of knowing who paid for the barrage.

The ads amount to the next wave of opposition to Obama’s health-care plan, which was upheld by the Supreme Court last month as constitutional under the federal government’s taxing authority. Some of the groups most active on the issue have received funding from health-care firms opposed to parts of the legislation.

The American Action Network (AAN), for example, is targeting House Democrats who voted for “Obamacare” with $1.2 million in digital ads, mailings and other efforts. As a tax-exempt “social welfare” organization, the group is not required to identify its funders to the public.

“How far will they go to protect Obama’s agenda?” a narrator asks in one digital ad, which equates a trio of New York Democrats with the Three Stooges comedy troupe. “Tell congressmen Bill Owens, Tim Bishop and Louise Slaughter to repeal the health care tax.”

Two donations to the group have emerged publicly from the health-care sector: $3 million in 2011 from insurance giant Aetna and $4.5 million in 2010 from the Pharmaceutical Research and Manufacturers Association (PhRMA), Washington’s biggest drugmaker lobby.

Aetna and PhRMA officials say their contributions were made in support of general policies, not political ads.

The effort underscores the secrecy that pervades many of the political ads inundating voters this year, particularly in a dozen crucial swing states that are likely to decide the presidential race between Obama and Republican challenger Mitt Romney.

The spending is part of a broader debate over whether companies and trade groups should be required to publicly reveal their funding sources, particularly in the wake of court rulings that have freed corporations to spend unlimited money on politics. Democrats, who are bearing the brunt of the attack ads by independent groups, failed in attempts last week to pass disclosure legislation aimed at forcing some nonprofits to reveal their donors.

“So many of these ads are cloaked in secrecy,” said Bruce F. Freed, president of the Center for Political Accountability, which lobbies corporations to disclose their political expenditures. “In a democracy, the public needs to know who is providing the funding, who the players are, whether you have folks engaging in duplicity or hypocrisy. Are the companies saying one thing but doing the opposite secretly?”

Some corporate political spending does spill into public view from time to time, often to the consternation of the firms doling out the money. In 2009, even as they publicly pledged support for elements of Obama’s health-care reform efforts, a group of major health-care firms funneled $86 million to the Chamber of Commerce for attack ads and lobbying efforts against the legislation, according to leaked records and news reports.

Since early 2010, candidates and outside groups have spent more than $235 million on negative ads about the law, according to data from Kantar Media/CMAG, which tracks ad spending. Many conservative groups say they plan to continue the push through November by making the health-care law a key focus of attack ads against Democrats.

Americans for Prosperity (AFP), a conservative group that has strong ties to the billionaire brothers who run Koch Industries, launched a $9 million anti-Obamacare ad campaign that began after the Supreme Court ruling and ran through mid-July. The ads ran in 12 swing states, including new targets for the group such as Minnesota.

“There’s going to be a lot more,” said AFP President Tim Phillips. “There’s nothing more important than repealing the health-care legislation in 2013. We want to be sure the public gets a sense of how important this is.”

Very little is known about where AFP gets its funding, which Phillips said is expected to top $70 million for 2012. “We don’t comment on any amounts that any particular donors give,” he said.

Another group running anti-Obamacare spots is a neworganization called American Commitment, which was founded last spring by Phil Kerpen, a longtime conservative activist who recently left AFP. The group has run more than $4 million worth of ads over the past month, including purchases of more than $1 million each targeting Sens. Bill Nelson (D-Fla.) and Sherrod Brown (D-Ohio).

Kerpen, who declined to identify the group’s funders, said earlier this month: “We’re going to spend as much as we can raise.”

Concerned Women for America, a Christian conservative group that usually focuses on social issues such as abortion, also leaped into the Obamacare fray last month with a $6  million ad buy in six states alleging that women would be denied care under the law. As a nonprofit, the group does not reveal its donors.

“This was something that people were talking about that we wanted to take a stand on,” said spokeswoman Alice Stewart.

Few groups on the left have been running ads in favor of the law since the Supreme Court ruling, and it’s not clear whether the issue will become a centerpiece of the Obama or Romney campaigns. But Jesse Ferguson, spokesman for the Democratic Congressional Campaign Committee, argues that the political landscape has shifted enough that many of the party’s House candidates will happily champion the legislation as part of their campaigns.

“The more special interest groups funded by health-insurance company profits choose to campaign against health insurance reforms, the more voters know those patient protections are important for the middle class and seniors,” Ferguson said.

In another ad campaign announced last week, the U.S. Chamber bought $1.1 million in airtime attacking Rep. Shelly Berkley (D-Nev.), a candidate for Senate, in part for backing health-care reform. The trade organization has strong backing from health-care firms, including $4.05 million from Aetna in 2011, according to documents revealed earlier this year by a trade publication.

Aetna spokeswoman Cynthia B. Michener said the company’s donations in 2011 to the Chamber and AAN were intended to spur debate on economic issues, not health care. She said the company has not made similar donations this year “due to the intensity of the political cycle.”

“The donations we made in 2011 helped fund educational programs focused on our economy,” Michener said. “Our goal in supporting organizations in 2011 was to raise awareness and build support for policies that will speed up the economic recovery and restart job growth.”

PhRMA’s tax records show that it gave more than $9 million to conservative advocacy groups in 2010 that actively opposed Obama’s health-care reforms, including the payment to AAN. The funding came after PhRMA had agreed to publicly support the legislation in 2009 in exchange for limits on the financial impact to the drug industry.

Senior vice president Matthew Bennett said in a statement that the group “contributes to organizations that share PhRMA’s goals of improving the quality of patients’ lives” and supporting new medical treatments and research.

AAN spokesman Dan Conston said the health-care law is “one of a number of key issues we intend to focus on this cycle.” In addition to its digital ad campaign, the group co-hosted an anti-Obamacare conference earlier this month with Crossroads GPS, a conservative nonprofit that has spent more than $18 million on ads opposing the law since 2010, according to the CMAG data.

The AAN and its affiliated super PAC, the Congressional Leadership Fund, have also reserved $6 million in fall advertising time to help a dozen House GOP candidates.

AAN, formed by former senator Norm Coleman (R-Minn.) and top Romney fundraiser Fred Malek, relies heavily on a small number of well-heeled donors for its funding, according to tax records.

Conston declined to say how much backing AAN has received from the health-care industry. “We really do not comment on donors,” he said.

Rachel Weiner contributed to this report.